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:7:1: error: expected identifier or '(' before 'return'
    7 | return  0;
      | ^~~~~~

[Hydro](https://hydro.ac)提供评测服务