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:8:22: error: expected ‘;’ before ‘for’
    8 |                 cnt=0
      |                      ^
      |                      ;
    9 |                 for(i=1;i<=n;i++)
      |                 ~~~   
foo.c:5:13: warning: variable ‘cnt’ set but not used [-Wunused-but-set-variable]
    5 |     int n,i,cnt;
      |             ^~~
foo.c:5:11: warning: unused variable ‘i’ [-Wunused-variable]
    5 |     int n,i,cnt;
      |           ^