/in/foo.c:2:1: error: unknown type name 'usingn'
    2 | usingn namespace std;
      | ^~~~~~
/in/foo.c:2:18: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    2 | usingn namespace std;
      |                  ^~~
/in/foo.c: In function 'main':
/in/foo.c:6:9: error: 'cin' undeclared (first use in this function)
    6 |         cin>>a>>b;
      |         ^~~
/in/foo.c:6:9: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:9: error: 'cout' undeclared (first use in this function)
    7 |         cout<<a+b<<endl;
      |         ^~~~
/in/foo.c:7:20: error: 'endl' undeclared (first use in this function)
    7 |         cout<<a+b<<endl;
      |                    ^~~~