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 in=new Scanner(System.in);
		int [] a=new int[6];
		for(int i=1;i<=5;i++){
			a[i]=in.nextInt();
		}
		int sum=0;
		for(int i=1;i<=5;i++){
			sum+=a[i];
		}
		if(sum%5!=0) System.out.println(-1);
		else System.out.println(sum/5);
	}
}

 			 		 					   			 	 			