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.ArrayList;
import java.util.Arrays;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Scanner;

public class Main {

	public static void main(String[] args){
		
		Scanner sc = new Scanner(System.in);
		
		int array[] = new int[5];
		
		int sum = 0;
		for(int i = 0; i < array.length; i++){
			array[i] = sc.nextInt();
			sum += array[i];
		}
		if(sum == 0){
			System.out.println("-1");
		}
		else if(sum % 5 == 0){
			int init = sum / 5;
			System.out.println(init);
		}
		else
			System.out.println("-1");
		sc.close();
	}
	
}