import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
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]);
            }
            double p = 1D / (N * (N - 1) / 2);

            double P[] = new double[max];

            Arrays.sort(A);
            for (int i = 0; i < N; i++) {
                for (int j = i + 1; j < N; j++) {
                    P[A[j] - A[i]] += p;
                }
            }

            double suffix[] = new double[max + 1];
            suffix[max] = 0D;
            for (int i = max - 1; i >= 0; i--) {
                suffix[i] = suffix[i + 1] + P[i];
            }

            double ans = 0D;
            for (int r1 = 1; r1 < max; r1++) {
                for (int r2 = 1; r1 + r2 < max; r2++) {
                    ans += P[r1] * P[r2] * suffix[r1 + r2 + 1];
                }
            }

//        for (int r1 = 1; r1 < max; r1++) {
//            for (int r2 = 1; r2 < max; r2++) {
//                for (int jerry = r1 + r2 + 1; jerry < max; jerry++) {
//                    if (P[r1] > 0 && P[r2] > 0 && P[jerry] > 0) {
//                        ans += P[r1] * P[r2] * P[jerry];
//                    }
//                }
//            }
//        }

            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());
        }

    }
}

