foo.c:2:1: error: unknown type name 'using'
 using namespace std;
 ^~~~~
foo.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。