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 c[6], b, mn = 101, mx, d;
bool found;
int main() {
  for (int i = 1; i <= 5; i++) {
    cin >> c[i];
    mx = max(mx, c[i]);
    mn = min(mn, c[i]);
  }
  for (int i = mn; i <= mx; i++) {
    d = 0;
    b = i;
    for (int j = 1; j <= 5; j++) d += i - c[j];
    if (d == 0) {
      found = true;
      break;
    }
  }
  if (found && b != 0)
    cout << b << endl;
  else
    cout << -1 << endl;
  return 0;
}
