import java.util.HashMap;
import java.util.Scanner;
public class Main {
public static void main(String[] args) {
Scanner scn = new Scanner(System.in);
int n = scn.nextInt();
int[] arr = new int[n + 1];
long sum = 0;
HashMap<Integer, Integer> hp = new HashMap<>();
for (int i = 1; i <= n; i++) {
int item = scn.nextInt();
arr[i] = arr[i - 1] + i;
if (hp.containsKey(item))
arr[i] -= hp.get(item);
hp.put(item, i);
sum += arr[i];
}
float ans = (float) (2 * sum - n) / (n * n);
System.out.println(ans);
}
}