#include <iostream>
using namespace std;
int main() {
//size of array
int n = 3;
int a[] = { 1, 2, 3 };
int i = 0;
long sum = 0;
for (i = 0; i < n; i++) {
sum += a[i];
}
long x = sum / n;
if (x * n == sum)
cout<<x<<endl;
else
cout<v("-1")<<endl;
return 0;
}
