import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.StringTokenizer;


public class D {

	public static void main(String[] args) throws IOException 
	{
		Scanner sc = new Scanner(System.in);
		PrintWriter out = new PrintWriter(System.out);
		
		int n = sc.nextInt();
		int[] a = new int[n];
		for(int i = 0; i < n; ++i)
			a[i] = sc.nextInt();
		int[] L = new int[5000], Z = new int[5000];
		int set1 = 0, set2 = 0;
		for(int i = 0; i < n; ++i)
			for(int j = 0; j < n; ++j)
				if(i != j)
					if(a[i] > a[j])
					{
						++set1;
						++L[a[i] - a[j]];
					}
					else
					{
						++set2;
						++Z[a[j] - a[i]];
					}
		
		long[] LL = new long[10000];
		for(int i = 0; i < 5000; ++i)
			for(int j = 0; j < 5000; ++j)
				LL[i + j] += (long)L[i] * L[j];
			
		
		for(int i = 1; i < 10000; ++i)
			LL[i] += LL[i-1];
		
		
		long den = (long)set1 * set1 * set2;
		long num = 0;
		for(int i = 1; i < 5000; ++i)
		{	
			
			num += (long)Z[i] * LL[(i - 1)/ 2];
		
		}
//		System.out.println(num);
//		System.out.println(den);
		out.println((double) num / den);
		out.flush();
		out.close();

	}
	
	
	static class Scanner 
	{
		StringTokenizer st;
		BufferedReader br;

		public Scanner(InputStream s){	br = new BufferedReader(new InputStreamReader(s));}

		public String next() throws IOException 
		{
			while (st == null || !st.hasMoreTokens()) 
				st = new StringTokenizer(br.readLine());
			return st.nextToken();
		}

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

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

		public String nextLine() throws IOException {return br.readLine();}

		public double nextDouble() throws IOException
		{
			String x = next();
			StringBuilder sb = new StringBuilder("0");
			double res = 0, f = 1;
			boolean dec = false, neg = false;
			int start = 0;
			if(x.charAt(0) == '-')
			{
				neg = true;
				start++;
			}
			for(int i = start; i < x.length(); i++)
				if(x.charAt(i) == '.')
				{
					res = Long.parseLong(sb.toString());
					sb = new StringBuilder("0");
					dec = true;
				}
				else
				{
					sb.append(x.charAt(i));
					if(dec)
						f *= 10;
				}
			res += Long.parseLong(sb.toString()) / f;
			return res * (neg?-1:1);
		}

		public boolean ready() throws IOException {return br.ready();}


	}
}