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.io.*;
import java.lang.reflect.Array;
public class acming {
	public static void main(String [] args) throws java.lang.Exception {
		BufferedReader bf = new BufferedReader( new InputStreamReader(System.in));
	   String [] s = bf.readLine().split(" ");
	   int a = Integer.parseInt(s[0])+Integer.parseInt(s[1])+Integer.parseInt(s[2])+Integer.parseInt(s[3])+
			   Integer.parseInt(s[4]) ;
	   if (a%5==0)
	          System.out.print(a/5);
	   else
		   System.out.print("-1");
}}
		
		
	
