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;
typedef pair<int, int> pii;
typedef vector<double> vd;
typedef vector<vector<bool>> vvb;
typedef vector<vector<string>> vvs;
typedef vector<vector<long long>> vvl;
template <class T>
inline istream& read(vector<T>& v) {
  for (auto& p : v) cin >> p;
  return cin;
}
template <class T>
inline istream& read(vector<vector<T>>& v) {
  for (auto& p : v) read(p);
  return cin;
}
int inf = 0x3f3f3f3f;
double eps = 1e-8;
long long mod = 1000000007ll;
struct FastIO {
  FastIO() {
    cin.sync_with_stdio(0);
    cin.tie(0);
  }
};
FastIO fio;
int main() {
  int total = 0, v;
  for (int i = (0); i < int(5); i++) {
    cin >> v;
    total += v;
  }
  printf("%d", total % 5 ? -1 : total / 5), printf("\n");
}
