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:5:1: error: expected ';', ',' or ')' before '{' token
    5 | {
      | ^

[Hydro](https://hydro.ac)提供评测服务