/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:7:24: error: 'endl' undeclared (first use in this function)
    7 |         cout << a+b << endl;
      |                        ^~~~