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 Codeforces273 {

    public static void main(String[] args){
        Scanner in = new Scanner(System.in);
        int c1 = in.nextInt();
        int c2 = in.nextInt();
        int c3 = in.nextInt();
        int c4 = in.nextInt();
        int c5 = in.nextInt();
        int res = c1 + c2 + c3 + c4 + c5;
        if (res % 5 == 0){
            System.out.println(res / 5);
        }
        else{
            System.out.println("-1");
        }
//        int m  = in.nextInt();
//        int[] digits = new int[n];
//        for (int i = 0; i < n ; i++) {
//            digits[i] = in.nextInt();
//        }
//        System.out.println("");
    }
}
