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.*;
import java.util.*;

public class cf273A {
    public static void main(String[] args) throws Exception {
    	// BufferedReader in = new BufferedReader(new FileReader("cf273A.in"));
    	BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
    	
		String[] arr = in.readLine().split(" ");
		
		int c1 = Integer.parseInt(arr[0]);		
		int c2 = Integer.parseInt(arr[1]);		
		int c3 = Integer.parseInt(arr[2]);		
		int c4 = Integer.parseInt(arr[3]);		
		int c5 = Integer.parseInt(arr[4]);
		int sum = c1 + c2 + c3 + c4 + c5;
		if(sum % 5 == 0 && sum != 0) {
			System.out.println(sum / 5);
		} else {
			System.out.println(-1);
		}
	}
}