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.io.BufferedReader;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.PriorityQueue;
import java.util.Queue;
import java.util.Scanner;
import java.util.StringTokenizer;
import java.util.concurrent.LinkedBlockingDeque;

import javax.swing.border.Border;

public class a {

	public static long mod = (long) Math.pow(10, 9) + 7;

	private static class node {
		int x;
		int y;

		node(int x, int c) {
			this.x = x;
			this.y = c;

		}

	}

	public static void main(String[] args) throws IOException {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		StringBuilder qq = new StringBuilder();
		PrintWriter out = new PrintWriter(new OutputStreamWriter(System.out));
		// int n=Integer.parseInt(in.readLine());
		String y[] = in.readLine().split(" ");
		int a[] = new int[5];
		int sum=0;
		
		for (int i = 0; i < 5; i++) {
			a[i] = Integer.parseInt(y[i]);
			sum+=a[i];
		}
		
		if(sum%5==0){
			
			for (int i = 0; i < a.length; i++) {
				if(a[i]==sum/5){
					System.out.println(-1);
					return ;
				}
					
			}
			System.out.println(sum/5);
			
		}
		else
			System.out.println(-1);
		// int n=Integer.parseInt(y[0]);

		out.close();

	}
}