Problem: You are given an array a consisting of n positive integers. You pick two integer numbers l and r from 1 to n, inclusive (numbers are picked randomly, equiprobably and independently). If l > r, then you swap values of l and r. You have to calculate the expected value of the number of unique elements in segment of the array from index l to index r, inclusive (1-indexed).

Input

The first line contains one integer number n (1 ≤ n ≤ 106). The second line contains n integer numbers a1, a2, ... an (1 ≤ ai ≤ 106) — elements of the array.

Output

Print one number — the expected number of unique elements in chosen segment. 

Your answer will be considered correct if its absolute or relative error doesn't exceed 10 - 4 — formally, the answer is correct if <image>, where x is jury's answer, and y is your answer.

Examples

Input

2
1 2


Output

1.500000


Input

2
2 2


Output

1.000000

Solution (in JAVA):

Solution:
import java.util.*;
import java.math.*;
import java.io.*;
import java.text.*;

public class A {

    //public static PrintWriter pw;
    public static PrintWriter pw = new PrintWriter(System.out);

    public static void solve() throws IOException {
//	pw=new PrintWriter(new FileWriter("C:\\Users\\shree\\Downloads\\small_output_in"));
        FastReader sc = new FastReader();
        int n=sc.I();
        int a[]=new int[n+1];
        ArrayList<Integer> v[]=new ArrayList[1000001];
        for(int i=1;i<=1e6;i++) v[i]=new ArrayList<>();
        for(int i=1;i<=n;i++){
            a[i]=sc.I();
            v[a[i]].add(i);
        }
        long ans=0;
        long d=n;
        long total=(d*(d+1))-d;
        for(int i=1;i<=1e6;i++){
            int prev=0;
            long ss=0;
            for(int u : v[i]){
                long l=u-prev-1;

                ss=ss+(l*(l+1))-l;
                prev=u;
            //    pw.println(u+" "+ss);
            }
            long l=n-prev;

                ss=ss+(l*(l+1))-l;


            ans=ans+total-ss;
         //   if(v[i].size()>0)
           // pw.println(ans+" "+l+" "+prev+" "+(total-ss));
        }

        pw.println(ans/(d*d*1.0));

        pw.close();


    }

    public static void main(String[] args) {
        new Thread(null, new Runnable() {
            public void run() {
                try {

                    solve();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        }, "1", 1 << 26).start();

    }

    static BufferedReader br;
    static long M = (long) 1e9 + 7;

    static class FastReader {

        StringTokenizer st;

        public FastReader() throws FileNotFoundException {
            //br=new BufferedReader(new FileReader("C:\\Users\\shree\\Downloads\\B-small-practice.in"));
            br = new BufferedReader(new InputStreamReader(System.in));

        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

        int I() {
            return Integer.parseInt(next());
        }

        long L() {
            return Long.parseLong(next());
        }

        double D() {
            return Double.parseDouble(next());
        }

        String nextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

        public boolean hasNext() throws IOException {
            while (st == null || !st.hasMoreTokens()) {
                String s = br.readLine();
                if (s == null) {
                    return false;
                }
                st = new StringTokenizer(s);
            }
            return true;
        }

    }
}