import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Artem Gilmudinov
 */
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.readInt();
            int[] a = new int[n];
            for (int i = 0; i < n; i++) {
                a[i] = in.readInt();
            }

            Arrays.sort(a);
            double total = (n - 1) * n / 2;
            int max = a[n - 1] + 1;
            double[] p1 = new double[max];
            double[] p2 = new double[2 * max];
            for (int i = 1; i < n; i++) {
                for (int j = i - 1; j >= 0; j--) {
                    ++p1[a[i] - a[j]];
                }
            }
            for (int i = 1; i < max; i++) {
                for (int j = 1; j < max; j++) {
                    p2[j + i] += p1[j] * p1[i];
                }
            }
            for (int i = 1; i < p2.length; i++) {
                p2[i] += p2[i - 1];
            }
            double total2 = total * total;
            for (int i = 0; i < p2.length; i++) {
                p2[i] /= total2;
            }
            double ans = 0;
            int diff;
            for (int i = 1; i < n; i++) {
                for (int j = i - 1; j >= 0; j--) {
                    diff = a[i] - a[j];
                    ans += p2[diff - 1];
                }
            }
            ans /= total;
            out.println(String.format("%.15f", ans));
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public int read() {
            if (numChars == -1)
                throw new InputMismatchException();
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0)
                    return -1;
            }
            return buf[curChar++];
        }

        public int readInt() {
            int c = read();
            while (isSpaceChar(c))
                c = read();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9')
                    throw new InputMismatchException();
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public boolean isSpaceChar(int c) {
            if (filter != null)
                return filter.isSpaceChar(c);
            return isWhitespace(c);
        }

        public static boolean isWhitespace(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }
}

