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 P478A
{
    public static void main (String[] args)
    {
        Scanner sc = new Scanner(System.in);
        
        int summ=0;
        
        for (int x=0; x<5; x++) {
            summ += sc.nextInt();
        }
        
        if (summ % 5 == 0 && summ != 0) {
            System.out.print(summ/5);
        }
        else {
            System.out.print(-1);
        }
    }
}