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

import java.util.Scanner;

public class problemA {
    private static final Scanner in = new Scanner(System.in);

    public static void main(String[] args) {
        int t = 1;
//        t = in.nextInt();
        while (t != 0) {
            solve();
            t--;
        }
    }

    public static void solve() {
        int a = in.nextInt(), b = in.nextInt(), c = in.nextInt(), d = in.nextInt(), e = in.nextInt();
        if ((a + b + c + d + e) % 5 == 0) {
            System.out.println((a + b + c + d + e) / 5 != 0 ? (a + b + c + d + e / 5) : -1);
        } else {
            System.out.println(-1);
        }
    }
}

