/in/foo.c:2:1: error: unknown type name 'using'
 using namespace std;
 ^~~~~
/in/foo.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^~~
/in/foo.c: In function 'main':
/in/foo.c:5:2: error: 'cin' undeclared (first use in this function)
  cin>>n>>k;
  ^~~
/in/foo.c:5:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:5:7: error: 'n' undeclared (first use in this function)
  cin>>n>>k;
       ^
/in/foo.c:5:10: error: 'k' undeclared (first use in this function)
  cin>>n>>k;
          ^
/in/foo.c:6:2: error: 'cout' undeclared (first use in this function)
  cout<<n+k;
  ^~~~