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 TaskA {
    static Scanner in = new Scanner(System.in);
    public static void main(String[] args){
        int a[] = new int[5];
        int fark = 0;
        for(int i = 0; i < 5; i++){
            a[i] = in.nextInt();
        }
        int max = a[0],min = a[0];
        for(int i = 0; i < 5; i++){
            if(a[i] > max)
                max = a[i];
            if(a[i] < min)
                min = a[i];
        }
        if(max - min > 5){
            System.out.println(-1);
        }else{
            fark = (max / 2 ) + 1;
            System.out.println(fark);
        }

    }
}
