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 ; 
public class A478_InitialBet {

	public static void main(String[] args) {
		Scanner ss =new Scanner (System.in) ; 
		int one = ss.nextInt() ; 
		int two = ss.nextInt() ; 
		int three = ss.nextInt() ; 
		int four = ss.nextInt() ; 
		int five = ss.nextInt() ; 
		int sum = one + two + three + four + five ; 
		if (sum%5 == 0)
			System.out.println(sum/5);
		else 
			System.out.println(-1);
	}

}
