/in/foo.cc:2:17: error: 'st' is not a namespace-name
 using namespace st
                 ^~
/in/foo.cc:3:1: error: expected namespace-name before 'int'
 int main()
 ^~~
/in/foo.cc:3:1: error: expected ';' before 'int'
/in/foo.cc: In function 'int main()':
/in/foo.cc:6:1: error: 'cin' was not declared in this scope
 cin>>a>>b;
 ^~~
/in/foo.cc:6:1: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:75:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/iostream:60:18: note:   'std::cin'
   extern istream cin;  /// Linked to standard input
                  ^~~
/in/foo.cc:7:1: error: 'cout' was not declared in this scope
 cout<<a*b+a+b;
 ^~~~
/in/foo.cc:7:1: note: suggested alternative:
In file included from /usr/include/x86_64-linux-gnu/c++/6/bits/stdc++.h:75:0,
                 from /in/foo.cc:1:
/usr/include/c++/6/iostream:61:18: note:   'std::cout'
   extern ostream cout;  /// Linked to standard output
                  ^~~~