

import java.io.*;
import java.util.*;
public class q4 {
	public static void main(String[] args) throws Exception
	{
		InputReader in=new InputReader(System.in);
		PrintWriter pw=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);
		int[] diff=new int[5010];
		for(int i=0;i<n;i++)
		{
			for(int j=i+1;j<n;j++)
			{
				diff[a[j]-a[i]]++;
			}
		}
		int[] diff2=new int[10100];
		for(int i=1;i<=5000;i++)
		{
			for(int j=1;j<=5000;j++)
			{
				diff2[i+j]+=diff[i]*diff[j];
			}
		}
		int ans=0;
		for(int i=1;i<=10000;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+=(diff2[i]*(n-r));
				
			}
		}
		double temp=n*(n-1)/2;
		double aa=ans*1.0/temp;
		aa=aa/temp;
		aa=aa/temp;
		pw.println(aa);
		
		
		pw.close();
	}
	
	
	
	
	
	
	
	
	 static class InputReader {

		private InputStream stream;
		private byte[] buf = new byte[8192];
		private int curChar;
		private int 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 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);
		}
	}

}
