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;

/**
 * Created by yuu on 7/3/17.
 */
public class Problem478A {
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        int sum = 0;
        for (int i = 0; i < 5; i++){
            sum += sc.nextInt();
        }
        if (sum % 5 == 0 && sum != 0){
            System.out.println(sum / 5);
        }
        else {
            System.out.println(-1);
        }
    }
}
