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

//package CodeForces;

import java.util.Scanner;
import java.util.Vector;

public class CF_1 {
    public static void main(String args[])
    {
        Scanner sc = new Scanner(System.in);
        int sum = 0 ;
        Vector <Integer> v = new Vector<Integer>();
        for(int  i  = 0 ; i < 5 ; i++)
        {
            int c = sc.nextInt();
            v.add(c);
            sum += v.get(i);
        }
        //System.out.println(sum);
        if(sum%5 != 0)
        {
            System.out.println("-1");
        }
        else
            System.out.println((sum/5));
    }
}
