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 x[5];
  cin >> x[0] >> x[1] >> x[2] >> x[3] >> x[4];
  if ((x[0] + x[1] + x[2] + x[3] + x[4]) % 5 == 0 &&
      (x[0] + x[1] + x[2] + x[3] + x[4]) != 0)
    cout << (x[0] + x[1] + x[2] + x[3] + x[4]) / 5;
  else
    cout << -1;
  return 0;
}
