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 Problem478A {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		int a = scan.nextInt(), b = scan.nextInt(), c = scan.nextInt(), d = scan.nextInt(), e = scan.nextInt();
		int sum = a+b+c+d+e;
		
		if(sum==0){
			System.out.print("-1");
		}else if(sum%5==0){
			System.out.print(sum/5);
		}else{
			System.out.print("-1");
		}
	}
}