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:24:9: warning: implicit declaration of function 'scanf_s'; did you mean 'scanf'? [-Wimplicit-function-declaration]
   24 |         scanf_s("%d %d", &n1, &n2);
      |         ^~~~~~~
      |         scanf
foo.c:23:23: warning: unused variable 'a2' [-Wunused-variable]
   23 |         int n1, n2,a1,a2;
      |                       ^~
foo.c:23:20: warning: unused variable 'a1' [-Wunused-variable]
   23 |         int n1, n2,a1,a2;
      |                    ^~

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