99 100 100 100 100
 57 83 11 4 93
 99 99 99 99 99
 100 0 0 0 0
 0 1 2 3 4
 93 100 99 90 98
 87 38 19 33 100
 1 1 1 1 1
 0 0 0 0 1
 2 3 4 5 6
 1 2 1 2 3
 0 0 0 0 0
 100 100 100 100 100
 43 83 1 0 23
 43 83 1 100 23
 56 0 0 0 4
 99 98 98 99 100

-1
 -1
 99
 20
 2
 96
 -1
 1
 -1
 4
 -1
 -1
 100
 30
 50
 12
 -1

#include <bits/stdc++.h>
using namespace std;
int main() {
  int C1, C2, C3, C4, C5;
  cin >> C1 >> C2 >> C3 >> C4 >> C5;
  if (C1 + C2 + C3 + C4 + C5 == 0) {
    cout << "-1";
  } else if ((C1 + C2 + C3 + C4 + C5) % 5 == 0) {
    cout << ((C1 + C2 + C3 + C4 + C5) / 5);
  } else {
    cout << "-1";
  }
}
