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

public class D {
	InputStream is;

	int __t__ = 1;
	int __f__ = 0;
	int __FILE_DEBUG_FLAG__ = __f__;
	String __DEBUG_FILE_NAME__ = "src/T";

	FastScanner in;
	PrintWriter out;

	int MAX = 5010;
	int MAX2 = 10100;
	
	public void solve() {
		int n = in.nextInt();
		int[] a = in.nextIntArray(n);
		Arrays.sort(a);
		/*
		double naive = 0;
		double unit = 1.;
		unit /= (n * (n - 1)) / 2;
		unit /= (n * (n - 1)) / 2;
		unit /= (n * (n - 1)) / 2;		
		for (int i1 = 0; i1 < n; i1++) for (int i2 = i1 + 1; i2 < n; i2++) {
			for (int j1 = 0; j1 < n; j1++) for (int j2 = j1 + 1; j2 < n; j2++) {
				for (int k1 = 0; k1 < n; k1++) for (int k2 = k1 + 1; k2 < n; k2++) {
					if ((a[i2] - a[i1] + a[j2] - a[j1]) < (a[k2] - a[k1])) 
						naive += unit;
				}
			}
		}
		System.out.println("naive : " + naive);
		*/
		long[] cnt = new long[MAX];
		long[] cnt2 = new long[MAX2];
		
		for (int i = 0; i < n; i++) {
			for (int j = i + 1; j < n; j++) {
				cnt[a[j] - a[i]]++;
			}
		}
		for (int i = 1; i < MAX; i++) {
			for (int j = 1; j < MAX; j++) {
				cnt2[i+j] += cnt[i] * cnt[j];
			}
		}
		long[] sum2 = new long[MAX2+1];
		for (int i = 1; i <= MAX2; i++) {
			sum2[i] = sum2[i-1] + cnt2[i-1];
		}
		double res = 0;
		for (int i = 1; i < MAX; i++) {
			double val = sum2[i];
			val /= (n * (n - 1)) / 2;
			val /= (n * (n - 1)) / 2;
			val /= (n * (n - 1)) / 2;
			val *= cnt[i];
			res += val;
		}
		//System.out.println(Arrays.toString(cnt));
		//System.out.println(Arrays.toString(cnt2));
		//System.out.println(Arrays.toString(sum2));
		
		System.out.println(res);
	}

	public void run() {
		if (__FILE_DEBUG_FLAG__ == __t__) {
			try {
				is = new FileInputStream(__DEBUG_FILE_NAME__);
			} catch (FileNotFoundException e) {
				// TODO 自動生成された catch ブロック
				e.printStackTrace();
			}
			System.out.println("FILE_INPUT!");
		} else {
			is = System.in;
		}
		in = new FastScanner(is);
		out = new PrintWriter(System.out);

		solve();
	}

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

	public void mapDebug(int[][] a) {
		System.out.println("--------map display---------");

		for (int i = 0; i < a.length; i++) {
			for (int j = 0; j < a[i].length; j++) {
				System.out.printf("%3d ", a[i][j]);
			}
			System.out.println();
		}

		System.out.println("----------------------------");
		System.out.println();
	}

	public void debug(Object... obj) {
		System.out.println(Arrays.deepToString(obj));
	}

	class FastScanner {
		private InputStream stream;
		private byte[] buf = new byte[1024];
		private int curChar;
		private int numChars;

		public FastScanner(InputStream stream) {
			this.stream = stream;
			//stream = new FileInputStream(new File("dec.in"));

		}

		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++];
		}

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

		boolean isEndline(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++)
				array[i] = nextInt();

			return array;
		}

		int[][] nextIntMap(int n, int m) {
			int[][] map = new int[n][m];
			for (int i = 0; i < n; i++) {
				map[i] = in.nextIntArray(m);
			}
			return map;
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++)
				array[i] = nextLong();

			return array;
		}

		long[][] nextLongMap(int n, int m) {
			long[][] map = new long[n][m];
			for (int i = 0; i < n; i++) {
				map[i] = in.nextLongArray(m);
			}
			return map;
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++)
				array[i] = nextDouble();

			return array;
		}

		double[][] nextDoubleMap(int n, int m) {
			double[][] map = new double[n][m];
			for (int i = 0; i < n; i++) {
				map[i] = in.nextDoubleArray(m);
			}
			return map;
		}

		String next() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		String[] nextStringArray(int n) {
			String[] array = new String[n];
			for (int i = 0; i < n; i++)
				array[i] = next();

			return array;
		}

		String nextLine() {
			int c = read();
			while (isEndline(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndline(c));
			return res.toString();
		}
	}
}

