/in/foo.c:3:1: error: unknown type name 'using'
    3 | using namespace std;
      | ^~~~~
/in/foo.c:3:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    3 | using namespace std;
      |                 ^~~
/in/foo.c: In function 'main':
/in/foo.c:8:9: error: 'cin' undeclared (first use in this function)
    8 |         cin >> a >> b;
      |         ^~~
/in/foo.c:8:9: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:9:9: error: 'cout' undeclared (first use in this function)
    9 |         cout << a + b << endl;
      |         ^~~~
/in/foo.c:9:26: error: 'endl' undeclared (first use in this function)
    9 |         cout << a + b << endl;
      |                          ^~~~