View Compilation Error

0_0_38616961_4788.c:2:1: error: unknown type name 'using'
 using namespace std;
 ^~~~~
0_0_38616961_4788.c:2:17: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'std'
 using namespace std;
                 ^~~
0_0_38616961_4788.c:4:5: error: variably modified 'a' at file scope
 int a[maxn];
     ^