import java.util.*;
import java.io.*;

public class D {
    FastScanner in;
    PrintWriter out;

    public void solve() throws IOException {
        int n = in.nextInt();
//        int n = 2000;

        Random rnd = new Random();
        int[] a = new int[n];
        Arrays.setAll(a, i -> in.nextInt());
//        Arrays.setAll(a, i -> rnd.nextInt(5000) + 1);
        Arrays.sort(a);
        long[] dif1 = new long[5000];
        for (int i = 0; i < n; i++) {
            for (int k = i + 1; k < n; k++) {
                dif1[a[k] - a[i]]++;
            }
        }
        long[] dif2 = new long[5000];
        for (int i = 0; i < dif1.length; i++) {
            for (int k = 0; k < dif1.length - i; k++) {
                dif2[i + k] += dif1[i] * dif1[k];
            }
        }

        long[] dif2sum = new long[5001];
        for (int i = 0; i < 5000; i++) {
            dif2sum[i + 1] = dif2sum[i] + dif2[i];
        }
        long ans = 0;
        for (int i = 0; i < 5000; i++) {
            ans += dif2sum[i] * dif1[i];
        }
        /*
        for (int dif = 0; dif < dif2.length; dif++) {
            for (int i = 0; i < n; i++) {
                int c1 = a[i];
                int less_c2 = c1 + dif;
                int l = 0;
                int r = n;
                while (l < r - 1) {
                    int m = (l + r) / 2;
                    if (a[m] > less_c2) {
                        r = m;
                    } else {
                        l = m;
                    }
                }
                ans += dif2[dif] * (long) (n - r);
            }
        }
        */
        double div = n * (n - 1) / 2;
        out.println((double) ans / div / div / div);
    }

    public void run() {
        try {
            in = new FastScanner(System.in);
            out = new PrintWriter(System.out);
            Locale.setDefault(Locale.US);

            solve();

            out.close();
        } catch (IOException e) {
            e.printStackTrace();
        }
    }

    class FastScanner {
        BufferedReader br;
        StringTokenizer st;

        FastScanner(InputStream f) {
            br = new BufferedReader(new InputStreamReader(f));
        }

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

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

    public static void main(String[] arg) {
        new D().run();
    }
}