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 Main {

    public static Scanner leer = new Scanner(System.in);
    public static void main(String[] args) {
        int c1,c2,c3,c4,c5;
        c1 = leer.nextInt();
        c2 = leer.nextInt();
        c3 = leer.nextInt();
        c4 = leer.nextInt();
        c5 = leer.nextInt();
        int resultado = c1+c2+c3+c4+c5;
        if(resultado == 0){
            System.out.println("0");
        }
        else{
            if(resultado%5==0){
                System.out.println(resultado/5);
            }
            else{
                System.out.println("-1");
            }
        }
        
    }

}