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.*;
import java.lang.Math;

public class Calculator {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String ss;
        String[] mas = new String[100];
        int t = 0;
        ss = sc.nextLine();

        //ss = ss.trim();
        mas = ss.split(" ");
        int n = mas.length;
      

        for (int i = 0; i < n; i++)
            t = t + Integer.parseInt(mas[i]);

        if (t % n == 0)
            t = t / n;
        else
            t = -1;

        System.out.println(t);

    }
}