foo.c:3:1: error: unknown type name 'using'
    3 | using namespace std;
      | ^~~~~
foo.c:3:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
    3 | using namespace std;
      |                 ^~~
foo.c: In function 'main':
foo.c:9:9: warning: implicit declaration of function 'scanf_s'; did you mean 'scanf'? [-Wimplicit-function-declaration]
    9 |         scanf_s("%d",&a);
      |         ^~~~~~~
      |         scanf

[Hydro](https://hydro.ac)提供评测服务