import java.io.InputStream;
import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.util.InputMismatchException;
import java.io.BufferedWriter;
import java.io.PrintWriter;
import java.io.Writer;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.io.IOException;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Alex
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        TaskD solver = new TaskD();
        solver.solve(1, in, out);
        out.close();
    }
    static class TaskD {
        int MAX = 5001;

        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int n = in.readInt(); // 2000
            int[] a = IOUtils.readIntArray(in, n);
            int[] count = new int[MAX];
            for(int i = 0; i < n; i++) {
                for(int j = 0; j < n; j++) {
                    if(a[i] - a[j] > 0) count[a[i] - a[j]]++;
                }
            }
            int[] moar = new int[MAX + 1];
            for(int i = MAX - 1; i >= 0; i--) moar[i] = moar[i + 1] + count[i];
            long totalcount = 0, wincount = 0;
            for(int first = 1; first < MAX; first++) {
                for(int second = 1; second < MAX; second++) {
                    long occurs = (long) count[first] * count[second];
                    totalcount += (long) n * (n - 1) * occurs / 2;
                    if(first + second + 1 < moar.length) wincount += (long) moar[first + second + 1] * occurs;
                    if(first + second + 1 < moar.length && (long) moar[first + second + 1] * occurs > 0) {
//                    out.printLine(first, second, moar[first + second + 1] , occurs);
                    }
//                if (occurs > 0) out.printLine(first, second, occurs);
                }
            }
            out.printLine((double) wincount / totalcount);
        }

    }

    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);

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void print(Object... objects) {
            for(int i = 0; i < objects.length; i++) {
                if(i != 0)
                    writer.print(' ');
                writer.print(objects[i]);
            }
        }

        public void printLine(Object... objects) {
            print(objects);
            writer.println();
        }

        public void close() {
            writer.close();
        }

    }

    static class IOUtils {
        public static int[] readIntArray(InputReader in, int size) {
            int[] array = new int[size];
            for(int i = 0; i < size; i++)
                array[i] = in.readInt();
            return array;
        }

    }
}

