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.Arrays;
import java.util.Scanner;
public class Main {
	public static void main(String[] args){
		Scanner in = new Scanner(System.in);
		int a[] = new int[5];
		for(int i = 0;i < 5;i++) {
			int x = in.nextInt();
			a[i] = x;
		}
		
		Arrays.sort(a);
		
		while(a[4] != a[0]){
			a[4]--;
			a[0]++;			
			Arrays.sort(a);
			if(a[0] == a[4]){
				int i = 0;
				for(;i < a.length;i++){
					
					if(a[0] == a[1]) continue;
					else{
						System.out.println(-1);
						return;
					}
				}
				
				if(i == 5){
					System.out.println(a[0]);
					return;
				}
				
				
			}else{
				int i = 0;
				for(i = 0;i < a.length-1;i++){
					if(a[0] != a[i]){
						break;
					}
				}
				if(i == 4){
					System.out.println(-1);
					return;
				}
			}
		}
		System.out.println(a[4]);
	}
}