#include <bits/stdc++.h>   using namespace std;   int main() {     int n = 5;     int arr[5], sum = 0;     for (int i = 0; i < n; i++) {       cin >> arr[i];       sum += arr[i];     }     if (sum == 0)       cout << -1 << endl;     else if (sum % 5 != 0)       cout << -1 << endl;     else {       int s = sum / 5;       int a = 0, b = 0;       for (int i = 0; i < n; i++) {         if (arr[i] > s)           a += s - arr[i];         else           b += arr[i] - s;       }       if (a == b)         cout << s << endl;       else         cout << -1 << endl;     }   }   