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 zephyr on 10/16/14.
 */
public class AllCoins {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int n  = 5;
        int index = 0;
        int total = 0;
        while (index < n){
            total += scanner.nextInt();
            index++;
        }
        int result = (total % 5 == 0 && total != 0 )? total / 5: -1;
        System.out.println(result);


    }
}