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() {
  long int x, i, j, k, p, q, m, n, a, s = 0, z[100], u;
  double d;
  cin >> x >> i >> j >> k >> p;
  s = x + i + j + k + p;
  if (x != 0 || i != 0 || j != 0 || k != 0 || p != 0) {
    if ((s % 5) != 0)
      cout << "-1" << endl;
    else
      cout << s / 5 << endl;
  } else
    cout << "-1" << endl;
  return 0;
}
