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.*;
	import java.lang.*;
	import java.math.*;
	public class Practice {
		
		public static void main(String[] args) {
			Scanner sc = new Scanner(System.in);
			
				int c1 = sc.nextInt();
				int c2 = sc.nextInt();
				int c3 = sc.nextInt();
				int c4 = sc.nextInt();
				int c5 = sc.nextInt();
				int sum=c1+c2+c3+c4+c5;
				if(sum%5==0) {
					System.out.println(sum/5);
				}else {
					System.out.println(-1);
				}
			
			
			} 
	}
		