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

/**
 * 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]]++;
                }
            }
            long all = ArrayUtils.sumArray(count);
            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 += all * occurs;
                    if(first + second + 1 < moar.length) wincount += occurs * moar[first + second + 1];
                }
            }
            out.printLine((double) wincount / totalcount);
        }

    }

    static class ArrayUtils {
        public static long sumArray(int[] array) {
            return new IntArray(array).sum();
        }

    }

    static abstract class IntAbstractStream implements IntStream {

        public String toString() {
            StringBuilder builder = new StringBuilder();
            boolean first = true;
            for(IntIterator it = intIterator(); it.isValid(); it.advance()) {
                if(first) {
                    first = false;
                } else {
                    builder.append(' ');
                }
                builder.append(it.value());
            }
            return builder.toString();
        }


        public boolean equals(Object o) {
            if(!(o instanceof IntStream)) {
                return false;
            }
            IntStream c = (IntStream) o;
            IntIterator it = intIterator();
            IntIterator jt = c.intIterator();
            while(it.isValid() && jt.isValid()) {
                if(it.value() != jt.value()) {
                    return false;
                }
                it.advance();
                jt.advance();
            }
            return !it.isValid() && !jt.isValid();
        }


        public int hashCode() {
            int result = 0;
            for(IntIterator it = intIterator(); it.isValid(); it.advance()) {
                result *= 31;
                result += it.value();
            }
            return result;
        }

    }

    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 interface IntIterator {
        public int value() throws NoSuchElementException;

        public boolean advance();

        public boolean isValid();

    }

    static interface IntList extends IntReversableCollection {
        public abstract int get(int index);

        public abstract void removeAt(int index);

        default public IntIterator intIterator() {
            return new IntIterator() {
                private int at;
                private boolean removed;

                public int value() {
                    if(removed) {
                        throw new IllegalStateException();
                    }
                    return get(at);
                }

                public boolean advance() {
                    at++;
                    removed = false;
                    return isValid();
                }

                public boolean isValid() {
                    return !removed && at < size();
                }

                public void remove() {
                    removeAt(at);
                    at--;
                    removed = true;
                }
            };
        }

    }

    static class IntArray extends IntAbstractStream implements IntList {
        private int[] data;

        public IntArray(int[] arr) {
            data = arr;
        }

        public int size() {
            return data.length;
        }

        public int get(int at) {
            return data[at];
        }

        public void removeAt(int index) {
            throw new UnsupportedOperationException();
        }

    }

    static interface IntReversableCollection extends IntCollection {
    }

    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 interface IntCollection extends IntStream {
        public int size();

    }

    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;
        }

    }

    static interface IntStream extends Iterable<Integer>, Comparable<IntStream> {
        public IntIterator intIterator();

        default public Iterator<Integer> iterator() {
            return new Iterator<Integer>() {
                private IntIterator it = intIterator();

                public boolean hasNext() {
                    return it.isValid();
                }

                public Integer next() {
                    int result = it.value();
                    it.advance();
                    return result;
                }
            };
        }

        default public int compareTo(IntStream c) {
            IntIterator it = intIterator();
            IntIterator jt = c.intIterator();
            while(it.isValid() && jt.isValid()) {
                int i = it.value();
                int j = jt.value();
                if(i < j) {
                    return -1;
                } else if(i > j) {
                    return 1;
                }
                it.advance();
                jt.advance();
            }
            if(it.isValid()) {
                return 1;
            }
            if(jt.isValid()) {
                return -1;
            }
            return 0;
        }

        default public long sum() {
            long result = 0;
            for(IntIterator it = intIterator(); it.isValid(); it.advance()) {
                result += it.value();
            }
            return result;
        }

    }
}

