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

public class _8VCVentureCupqD {

	public static void main(String[] args) {
		InputReader in = new InputReader(System.in);
		PrintWriter w = new PrintWriter(System.out);

		int n = in.nextInt();

		int a[] = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = in.nextInt();
		
		Arrays.sort(a);
		
		long cnt1[] = new long[5010];
		for(int i = 0; i < n; i++)
			for(int j = i + 1; j < n; j++)
				cnt1[a[j] - a[i]]++;

		long cnt[] = new long[10030];
		for(int i = 1; i < cnt1.length; i++)
			for(int j = 1; j < cnt1.length; j++)
				cnt[i + j] += cnt1[i] * cnt1[j];
		
		long ans = 0;
		for(int i = 1; i < cnt.length; i++){
			int r = 0;
			for(int j = 0; j < n; j++){
				while(r < n && a[r] - a[j] <= i)
					r++;
				if(r == n)
					break;
				ans += cnt[i] * (n - r);
			}
		}
		
		long N = n;
		long tot = N * (N - 1) / 2;
		tot = tot * tot * tot;
		
		w.println(ans * 1.0 / tot);
		
		w.close();
	}

	static class InputReader {

		private InputStream stream;
		private byte[] buf = new byte[8192];
		private int curChar, snumChars;
		private SpaceCharFilter filter;

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

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

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

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

		public int[] nextIntArray(int n) {
			int a[] = new int[n];
			for (int i = 0; i < n; i++)
				a[i] = nextInt();
			return a;
		}

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

		public boolean isSpaceChar(int c) {
			if (filter != null)
				return filter.isSpaceChar(c);
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

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