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.Scanner;
import java.util.ArrayList;

public class InitialBet {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		ArrayList<Integer> numbers = new ArrayList<Integer>();
		int tmp;
		while (sc.hasNextInt()) {
			tmp = sc.nextInt();
			numbers.add(tmp);
		}

		tmp = 0;
		int counter = 0;
		for (int number : numbers) {
			tmp += number;
			counter++;		
		}

		if (tmp % counter != 0) {
			System.out.println("-1"); 
		} else {
			int average = tmp / counter;
			System.out.println(average); 
		}
	}

}