foo.cc:2:16: error: expected identifier before ';' token
    2 | using namespace;
      |                ^
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>>y1,m1,d1,y2,m2,d2;
      |  ^~~
      |  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:7:8: error: lvalue required as left operand of assignment
    7 |  y1-y2=y;
      |        ^
foo.cc:8:8: error: lvalue required as left operand of assignment
    8 |  m1-m2=m;
      |        ^
foo.cc:9:8: error: lvalue required as left operand of assignment
    9 |  d1-d2=d;
      |        ^
foo.cc:10:15: error: lvalue required as left operand of assignment
   10 |  365*y+30*m+d=n;
      |               ^
foo.cc:11:2: error: 'cout' was not declared in this scope; did you mean 'std::cout'?
   11 |  cout<<n;
      |  ^~~~
      |  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
      |                  ^~~~

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