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 InitialBet478A {

	public static void main(String[] args) {
		Scanner kyb = new Scanner(System.in);
		int T[]= new int[5];
		int somme=0;
		for(int i=0;i<T.length;i++) {
			T[i]=kyb.nextInt();
			somme+=T[i];

		}
		if(somme%5==0 ) {
			if(somme==0) {
				System.out.println(-1);
			}
			else {
				System.out.println(somme/5);
			}
		}
		else {
			System.out.println(-1);
		}



	}

}
