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

public class Solution {
    // TODO: do not forget
    private static final int READ_FROM_FILE = 0;
    private static final int WRITE_TO_FILE = 0;
    private static final String INPUT_FILE = "input.txt";
    private static final String OUTPUT_FILE = "output.txt";

    private void run(InputReader in, PrintWriter out) {
        int n = in.nextInt();
        int a[] = new int[n];
        for(int i = 0; i < n; ++i) {
            a[i] = in.nextInt();
        }
        int cnt[] = new int[5000];
        for(int i = 0; i < n; ++i) {
            for(int j = i + 1; j < n; ++j) {
                cnt[Math.abs(a[i] - a[j])]++;
            }
        }
        int sum[] = new int[5000];
        for(int i = 1; i < 5000; ++i) {
            sum[i] = sum[i - 1] + cnt[i];
        }

        long ans = 0;
        for(int d3 = 4999; d3 > 2; --d3) {
            if(cnt[d3] == 0) {
                continue;
            }
            for(int d2 = 1; d2 + 1 < d3; d2++) {
                if(cnt[d2] == 0) {
                    continue;
                }
                int d1 = d3 - d2 - 1;
                ans += cnt[d3] * cnt[d2] * sum[d1];
            }
        }

        long X = n * (n - 1) / 2;
        out.print(String.format("%.9f", (double)ans * 1.0 / (X * X * X)).replace(",", "."));
    }

    public static void main(String[] args) {
        InputReader in;
        PrintWriter out;
        try {
            if (READ_FROM_FILE == 1) {
                in = new InputReader(new FileInputStream(INPUT_FILE));
            } else {
                in = new InputReader(System.in);
            }
            if (WRITE_TO_FILE == 1) {
                out = new PrintWriter(OUTPUT_FILE);
            } else {
                out = new PrintWriter(System.out);
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }

        Solution solution = new Solution();
        solution.run(in, out);

        out.println();
        out.flush();
        out.close();
    }

    private static class InputReader {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream));
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

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

        public long nextLong() {
            return Long.parseLong(next());
        }

        public double nextDouble() {
            return Double.parseDouble(next());
        }

        public char nextCharacter() {
            return next().charAt(0);
        }
    }
}