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

// 478A - Initial Bet
// http://codeforces.com/problemset/problem/478/A
// Farwa Naqi

import java.util.*;
import java.lang.*;
import java.io.*;

public class InitialBet478A
{
	public static void main (String[] args) throws java.lang.Exception
	{
		// Get input
		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");
	}
}