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 Binarysearch {
    public static void main(String[] args) {
        Scanner input = new Scanner(System.in);
        int sum =0;
        int[] a = new int[5];
        for(int i=0;i<5;i++){
            a[i] = input.nextInt();
            sum+=a[i];
        }
        if(sum%5==0 && sum!=0) System.out.printf("%d\n",sum/5);
        else System.out.printf("%d\n",-1);
         
    }
}