View Compilation Error

0_0_31382415_14044.cpp:3:11: error: '::main' must return 'int'
 void main(){
           ^
0_0_31382415_14044.cpp: In function 'int main()':
0_0_31382415_14044.cpp:6:18: error: 'scanf_s' was not declared in this scope
  scanf_s("%d", &a);
                  ^