View Compilation Error

0_0_31386526_23851.cpp:71:11: error: '::main' must return 'int'
 void main() {
           ^
0_0_31386526_23851.cpp: In function 'int main()':
0_0_31386526_23851.cpp:73:18: error: 'scanf_s' was not declared in this scope
  scanf_s("%d", &a);
                  ^