/in/foo.c:2:1: error: unknown type name 'using'
    2 | using namespace std;
      | ^~~~~
/in/foo.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    2 | using namespace std;
      |                 ^~~
/in/foo.c: In function 'main':
/in/foo.c:7:2: error: 'cout' undeclared (first use in this function)
    7 |  cout<<"n="<<endl;
      |  ^~~~
/in/foo.c:7:2: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:14: error: 'endl' undeclared (first use in this function)
    7 |  cout<<"n="<<endl;
      |              ^~~~
/in/foo.c:8:2: error: 'cin' undeclared (first use in this function)
    8 |  cin>>n;
      |  ^~~