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>
int main() {
  int c1, c2, c3, c4, c5;
  while (scanf("%d%d%d%d%d", &c1, &c2, &c3, &c4, &c5) != EOF) {
    int sum = 0;
    sum = c1 + c2 + c3 + c4 + c5;
    if (sum % 5 == 0 && sum > 0)
      printf("%d\n", sum / 5);
    else
      printf("-1\n");
  }
  return 0;
}
