/in/foo.c:1:1: error: unknown type name 'using'
 using namespace std;
 ^~~~~
/in/foo.c:1:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^~~
/in/foo.c: In function 'main':
/in/foo.c:8:2: error: 'cin' undeclared (first use in this function)
  cin >> k;
  ^~~
/in/foo.c:8:2: note: each undeclared identifier is reported only once for each function it appears in