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: In function 'main':
foo.c:30:5: warning: implicit declaration of function 'scanf_s'; did you mean 'scanf'? [-Wimplicit-function-declaration]
   30 |     scanf_s("%d %d", &n1, &n2);
      |     ^~~~~~~
      |     scanf

[Hydro](https://hydro.ac)提供评测服务