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 void main(String[] args) {
        Scanner ler = new Scanner(System.in);
        int c1,c2,c3,c4,c5,t;
        c1=ler.nextInt();
        c2=ler.nextInt();
        c3=ler.nextInt();
        c4=ler.nextInt();
        c5=ler.nextInt();
        t=c1+c2+c3+c4+c5;
        if(c1<=0&&c1>100&&c2<=0&&c2>100&&c3<=0&&c3>100&&c4<=0&&c4>100&&c5<=0&&c5>100){
            System.out.println("-1");
        }else if(t%5!=0&&t<=0){
            System.out.println("-1");
        }else{
            System.out.println(t/5);
        }
    }
}
// 1506777495520
