import java.util.Scanner;
public class BooksWithOrphans {
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
int n = sc.nextInt();
int[] arr = new int[n];
int sum = 0;
for (int i = 0; i < n; i++) {
arr[i] = sc.nextInt();
sum += arr[i];
}
if (sum % n != 0) {
System.out.println(-1);
return;
}
int count = 0;
while (true) {
int min = Integer.MAX_VALUE, max = Integer.MIN_VALUE, minI = 0, maxI = 0;
for (int i = 0; i < n; i++) {
if (arr[i] < min) {
min = arr[i];
minI = i;
}
if (arr[i] > max) {
max = arr[i];
maxI = i;
}
}
if (max == min) {
break;
}
arr[maxI]--;
arr[minI]++;
count++;
}
System.out.println(count);
}
}
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