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

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Abhas Jain
 */
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.ni();
            int[] ar = new int[n];
            int max = 0;
            for (int i = 0; i < n; ++i) {
                ar[i] = in.ni();
                max = Math.max(max, ar[i]);
            }
            long[] diff = new long[max + 1];
            for (int i = 0; i < n; ++i) {
                for (int j = i + 1; j < n; ++j) {
                    diff[Math.abs(ar[i] - ar[j])]++;
                }
            }
            long ans = 0L;
            long[] prefix = new long[max + 1];
            prefix[max] = diff[max];
            for (int i = max - 1; i > -1; --i) {
                prefix[i] = prefix[i + 1] + diff[i];
            }
            for (int i = 1; i <= max; ++i) {
                for (int j = 1; i + j + 1 <= max; ++j) {
                    ans += (prefix[i + j + 1]) * diff[i] * diff[j];
                }
            }
            // out.print(ans);
            double s = (double) ans;
            for (int i = 0; i < 3; ++i) {
                s /= (n * (n - 1) / 2D);
            }
            out.print(s);
        }

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.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 ni() {
            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);

        }

    }
}

