foo.cc:2:7: error: expected nested-name-specifier before 'namesapce'
    2 | using namesapce std;
      |       ^~~~~~~~~
foo.cc: In function 'int main()':
foo.cc:6:2: error: 'cin' was not declared in this scope; did you mean 'std::cin'?
    6 |  cin>>a>>b>>c;
      |  ^~~
      |  std::cin
In file included from foo.cc:1:
/usr/include/c++/9/iostream:60:18: note: 'std::cin' declared here
   60 |   extern istream cin;  /// Linked to standard input
      |                  ^~~
foo.cc:9:10: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
    9 |          cout<<b<<a<<endl;
      |          ^~~~
      |          std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~
foo.cc:9:22: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
    9 |          cout<<b<<a<<endl;
      |                      ^~~~
      |                      std::endl
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:599:5: note: 'std::endl' declared here
  599 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
foo.cc:12:14: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   12 |              cout<<c<<a<<endl;
      |              ^~~~
      |              std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~
foo.cc:12:26: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   12 |              cout<<c<<a<<endl;
      |                          ^~~~
      |                          std::endl
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:599:5: note: 'std::endl' declared here
  599 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
foo.cc:14:14: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   14 |              cout<<a<<c<<endl;
      |              ^~~~
      |              std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~
foo.cc:14:26: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   14 |              cout<<a<<c<<endl;
      |                          ^~~~
      |                          std::endl
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:599:5: note: 'std::endl' declared here
  599 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
foo.cc:17:11: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   17 |           cout<<a<<b<<endl;
      |           ^~~~
      |           std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~
foo.cc:17:23: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   17 |           cout<<a<<b<<endl;
      |                       ^~~~
      |                       std::endl
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:599:5: note: 'std::endl' declared here
  599 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
foo.cc:20:15: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   20 |               cout<<c<<b<<endl;
      |               ^~~~
      |               std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~
foo.cc:20:27: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   20 |               cout<<c<<b<<endl;
      |                           ^~~~
      |                           std::endl
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:599:5: note: 'std::endl' declared here
  599 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~
foo.cc:22:15: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   22 |               cout<<b<<c<<endl;
      |               ^~~~
      |               std::cout
In file included from foo.cc:1:
/usr/include/c++/9/iostream:61:18: note: 'std::cout' declared here
   61 |   extern ostream cout;  /// Linked to standard output
      |                  ^~~~
foo.cc:22:27: error: 'endl' was not declared in this scope; did you mean 'std::endl'?
   22 |               cout<<b<<c<<endl;
      |                           ^~~~
      |                           std::endl
In file included from /usr/include/c++/9/iostream:39,
                 from foo.cc:1:
/usr/include/c++/9/ostream:599:5: note: 'std::endl' declared here
  599 |     endl(basic_ostream<_CharT, _Traits>& __os)
      |     ^~~~

[Hydro](https://hydro.ac)提供评测服务