import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.*;

public class Main {
    public static void main(String[] args) {
//     Test.testing();
        ConsoleIO io = new ConsoleIO();
        new Main(io).solve();
        io.flush();
    }
    ConsoleIO io;
    Main(ConsoleIO io) {
        this.io = io;
    }

    ArrayList<ArrayList<Integer>> gr1;
    boolean[] visit;
    //

    class Pair{
        public Pair(int a, int b){
            this.a = a;
            this.b = b;
        }
        public int a;
        public int b;
    }

    class Edge {
        public Edge(int c, int u,int v) {
            this.u = u;
            this.v = v;
            this.c = c;
        }
        public int u;
        public int v;
        public int c;
    }

    long MOD = 1_000_000_007;
    int n,m,k,fn;
    int[][] map;
    public void solve() {
        n = io.readInt();
        int[] l = io.readIntArray();
        int[] dif = new int[5001];
        for(int i = 0;i<l.length;i++)
            for(int j = i+1;j<l.length;j++) {
                dif[Math.abs(l[i] - l[j])]++;
            }
        int prev = 0;
        long tot = 0;
        for(int i = 1;i<dif.length;i++) {
            int t = 0;
            for (int j = 0; j <= i - 1; j++) {
                int k = i - 1 - j;
                t += dif[j] * dif[k];
            }
            prev += t;
            tot += (long) prev * dif[i];
        }
        double res = tot*1.0/(n*(n-1)/2)/(n*(n-1)/2)/(n*(n-1)/2);
        io.writeLine(res+"");
    }

    long gcd(long a, long b) {
        if (a < b) return gcd(b, a);
        if (b == 0) return a;
        return gcd(b, a % b);
    }
}

class ConsoleIO {
    BufferedReader br;
    PrintWriter out;
    public ConsoleIO(){br = new BufferedReader(new InputStreamReader(System.in));out = new PrintWriter(System.out);}
    public void flush(){this.out.close();}
    public void writeLine(String s) {this.out.println(s);}
    public void writeInt(int a) {this.out.print(a);this.out.print(' ');}
    public void writeWord(String s){
        this.out.print(s);
    }
    public String readLine() {try {return br.readLine();}catch (Exception ex){ return "";}}
    public long readLong() {
        return Long.parseLong(this.readLine());
    }
    public int readInt() {
        return Integer.parseInt(this.readLine().trim());
    }
    public long[] readLongArray() {
        String[]n=this.readLine().trim().split(" ");long[]r=new long[n.length];
        for(int i=0;i<n.length;i++)r[i]=Long.parseLong(n[i]);
        return r;
    }
    public int[] readIntArray() {
        String[]n=this.readLine().trim().split("\\s+");int[]r=new int[n.length];
        for(int i=0;i<n.length;i++)r[i]=Integer.parseInt(n[i]);
        return r;
    }
    public void writeIntArray(int[] a) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < a.length; i++) {if (i > 0) sb.append(' ');sb.append(a[i]);}
        this.writeLine(sb.toString());
    }
}


