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


public class Solution {
    private BufferedReader in;
    private StringTokenizer line;
    private PrintWriter out;

    private static final int mm = 1000000007;

    public void solve() throws IOException {
        int n = nextInt();
        int[] a = nextIntArray(n);
        Arrays.sort(a);
        int[] d = new int[5013];
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < i; j++) {
                d[a[i] - a[j]]++;
            }
        }
        long[] dm = new long[5013];
        for (int i = 5003; i >= 0; i--) {
            dm[i] = dm[i + 1] + d[i];
        }
        long[] dd = new long[10013];
        for (int i = 0; i <= 5003; i++) {
            for (int j = 0; j <= 5003; j++) {
                dd[i + j] += d[i] * d[j];
            }
        }
        long res = 0;
        for (int i = 0; i <= 5006; i++) {
            res += dm[i + 1] * dd[i];
        }
        long dv = n * (n - 1) / 2;
        dv = dv * dv * dv;
        out.println(String.format(Locale.ENGLISH, "%.10f", res / (double) dv));
    }

    public static void main(String[] args) throws IOException {
        new Solution().run(args);
    }

    public void run(String[] args) throws IOException {
        if (args.length > 0 && "DEBUG_MODE".equals(args[0])) {
            in = new BufferedReader(new InputStreamReader(new FileInputStream("input.txt")));
        } else {
            in = new BufferedReader(new InputStreamReader(System.in));
        }
        out = new PrintWriter(System.out);
//        out = new PrintWriter("output.txt");

//        int t = nextInt();
        int t = 1;
        for (int i = 0; i < t; i++) {
//            out.print("Case #" + (i + 1) + ": ");
            solve();
        }

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

    private int[] nextIntArray(int n) throws IOException {
        int[] res = new int[n];
        for (int i = 0; i < n; i++) {
            res[i] = nextInt();
        }
        return res;
    }

    private long[] nextLongArray(int n) throws IOException {
        long[] res = new long[n];
        for (int i = 0; i < n; i++) {
            res[i] = nextInt();
        }
        return res;
    }

    private int nextInt() throws IOException {
        return Integer.parseInt(nextToken());
    }

    private long nextLong() throws IOException {
        return Long.parseLong(nextToken());
    }

    private double nextDouble() throws IOException {
        return Double.parseDouble(nextToken());
    }

    private String nextToken() throws IOException {
        while (line == null || !line.hasMoreTokens()) {
            line = new StringTokenizer(in.readLine());
        }
        return line.nextToken();
    }

    private static class Pii {
        private int key;
        private int value;

        public Pii(int key, int value) {
            this.key = key;
            this.value = value;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            Pii pii = (Pii) o;

            if (key != pii.key) return false;
            return value == pii.value;

        }

        @Override
        public int hashCode() {
            int result = key;
            result = 31 * result + value;
            return result;
        }
    }

    private static class Pair<K, V> {
        private K key;
        private V value;

        public Pair(K key, V value) {
            this.key = key;
            this.value = value;
        }

        public K getKey() {
            return key;
        }

        public V getValue() {
            return value;
        }

        @Override
        public boolean equals(Object o) {
            if (this == o) return true;
            if (o == null || getClass() != o.getClass()) return false;

            Pair<?, ?> pair = (Pair<?, ?>) o;

            if (key != null ? !key.equals(pair.key) : pair.key != null) return false;
            return !(value != null ? !value.equals(pair.value) : pair.value != null);

        }

        @Override
        public int hashCode() {
            int result = key != null ? key.hashCode() : 0;
            result = 31 * result + (value != null ? value.hashCode() : 0);
            return result;
        }
    }
}
