/in/foo.c:1:1: error: unknown type name 'using'
    1 | using namespace std;
      | ^~~~~
/in/foo.c:1:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    1 | using namespace std;
      |                 ^~~
/in/foo.c: In function 'main':
/in/foo.c:6:5: error: 'cin' undeclared (first use in this function)
    6 |     cin
      |     ^~~
/in/foo.c:6:5: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:8:5: error: 'cout' undeclared (first use in this function)
    8 |     cout
      |     ^~~~
/in/foo.c:9:13: error: 'endl' undeclared (first use in this function)
    9 | << a + b << endl;
      |             ^~~~