foo.c:2:1: error: unknown type name ‘using’
    2 | using namespace std;
      | ^~~~~
foo.c:2:17: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘std’
    2 | using namespace std;
      |                 ^~~
foo.c: In function ‘main’:
foo.c:7:9: error: ‘cin’ undeclared (first use in this function)
    7 |         cin>>a;
      |         ^~~
foo.c:7:9: note: each undeclared identifier is reported only once for each function it appears in
foo.c:17:9: error: ‘cout’ undeclared (first use in this function)
   17 |         cout<<i<<endl;
      |         ^~~~
foo.c:17:18: error: ‘endl’ undeclared (first use in this function)
   17 |         cout<<i<<endl;
      |                  ^~~~