分苹果

import java.util.Scanner;

public class Main1 {
	public static void main(String[] args) {
		Scanner scanner=new Scanner(System.in);
		int n=scanner.nextInt();
		int [] a=new int[n];	
		int sum=0;
		for(int i=0;i<n;i++){
			a[i]=scanner.nextInt();
			sum+=a[i];
		}
		if (sum%n!=0) {
			System.out.println(-1);
			return;
		}
		
		int x=sum/n;
		for (int i = 0; i < a.length; i++) {
			if (Math.abs(a[i]-x)%2!=0) {
				System.out.println(-1);
				return;
			}
			
		}
		int cout=0;
		for (int i = 0; i < a.length; i++) {
			if (a[i]<x) {
				cout+=(x-a[i])/2;
			}
		}
		System.out.println(cout);
	}
}