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 mult(int x, int y) {
  long long ans, x1 = (long long)x, y1 = (long long)y;
  ans = (x1 * y1) % 1000000007;
  return (int)ans;
}
int gcd(int a, int b) { return b == 0 ? a : gcd(b, a % b); }
int lcm(int a, int b) { return a * (b / gcd(a, b)); }
int pow1(int a, int b) {
  int ans = 1;
  while (b > 0) {
    if (b & 1) ans = mult(ans, a);
    a = mult(a, a);
    b >>= 1;
  }
  return ans;
}
int mina(int arr[], int n) {
  int x = arr[0], i, pos = 0;
  for (i = 1; i < n; i++) {
    if (arr[i] < x) {
      x = arr[i];
      pos = i;
    }
  }
  return x;
}
int maxa(int arr[], int n) {
  int x = arr[0], i, pos = 0;
  for (i = 1; i < n; i++) {
    if (arr[i] > x) {
      x = arr[i];
      pos = i;
    }
  }
  return x;
}
int main() {
  long long sum = 0;
  for (int i = 0; i < 5; i++) {
    int k;
    cin >> k;
    sum += k;
  }
  if (sum % 5 != 0)
    cout << "-1" << '\n';
  else
    cout << sum / 5 << '\n';
  return 0;
}
