import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.TreeMap;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskD solver = new TaskD();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskD {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int N = in.nextInt();

            int A[] = new int[N];
            int max = 0;
            for (int i = 0; i < N; i++) {
                A[i] = in.nextInt();
                max = Math.max(max, A[i]);
            }
            TreeMap<Integer, Integer> map = new TreeMap<>();
            int size = 0;
            for (int i = 0; i < N; i++) {
                for (int j = 0; j < N; j++) {
                    if (i == j) continue;
                    int diff = A[i] - A[j];
                    if (diff > 0) {
                        map.compute(diff, (k, v) -> (v == null ? 1 : v + 1));
                        size++;
                    }
                }
            }

            int val[] = new int[map.keySet().size()];
            int cnt[] = new int[map.keySet().size()];
            int c = 0;
            for (Integer key : map.keySet()) {
                val[c] = key;
                cnt[c] = map.get(key);
                c++;
            }

            //P[X][Y] - probability that the difference of points after Y GAME is X;
            int MAX = 2 * max + 1;
            double P[][] = new double[4][MAX];
            for (int i = 0; i < 4; i++) {
                Arrays.fill(P[i], -30D);
            }
            P[0][MAX / 2] = 1D;
            int start = 0;
            int end = 0;
            int middle = MAX / 2;
            for (int i = 0; i < val.length; i++) {
                double rozn = cnt[i];
                P[1][middle - val[i]] = (rozn / size);
            }

            for (int game = 2; game <= 3; game++) {
                if (game < 3) {
                    start = 0;
                    end = middle;
                } else {
                    start = middle;
                    end = MAX - 1;
                }
                for (int diff = start; diff <= end; diff++) {
                    for (int i = 0; i < val.length; i++) {
                        int cur = val[i];
                        if (game < 3) {
                            cur = -cur;
                        }
                        if (diff - cur >= 0 && diff - cur <= end && P[game - 1][diff - cur] > -19) {
                            if (P[game][diff] < -19) {
                                P[game][diff] = 0D;
                            }
                            double rozn = cnt[i];

                            P[game][diff] += P[game - 1][diff - cur] * (rozn / size);
                        }
                    }
                }
            }

            double ans = 0D;
            for (int diff = middle + 1; diff < MAX; diff++) {
                if (P[3][diff] > -19) {
                    ans += P[3][diff];
                }
            }

            out.println(ans);
        }

    }

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

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            tokenizer = null;
        }

        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());
        }

    }
}

