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 A273 {

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