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.Collections;
import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Main {
    public static void main(String args[]) {
        Scanner input = new Scanner(System.in);
        int sum=0,temp;
        for(int i=0 ;i<5;i++){
            temp=input.nextInt();
            sum+=temp;
            }
        if(sum%5==0&&sum!=0){
            System.out.println(sum/5);
        }else{
            System.out.println(-1);
        }
    }
}

