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.*;
public class exam2 {

	public static void main(String[] args) {
Scanner scanner = new Scanner (System.in);
		
		
		int n=0;
		int sum=0;
		while (n<5) {
			 int input = scanner.nextInt();
			 if(0<=input && input<=100) {
				sum=sum+input;
				n++;
			 }
			
			 else {
				 break;
			 }
		}
		
		
		if(n==5) {
			 if(n==5) {
				 if (sum==0 ||sum % 5 != 0)  System.out.println("-1");
					else System.out.println(sum/5);
			 }
		
		}
		
	
	

	}

}
