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

public class Main {
	
	public static void main(String[] args) throws Throwable {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		int[] a = sc.nexIntArray();
		Arrays.sort(a);
		int len = a[n-1];
		double[] pDif = new double[len];
		int total = ((n-1)*n)/2;
		double pr = 1.0/(total);
		for(int i = 0; i < n;i++)
		{
			for(int j = i+1;j<n;j++)
			{
				int dif = a[j]-a[i];
				pDif[dif] += pr;
			}
		}
		
		double[] preSum = new double[len];
		preSum[len-1] = pDif[len-1];
		for(int i = len-2;i>=0;i--)
			preSum[i] = pDif[i] + preSum[i+1];
		
		double ans = 0;
		
		for(int i = 1; i < len;i++)
		{
			for(int j = 1;j<len;j++)
			{
				int dif = i+j;
				if(dif+1>=len)continue;
				ans += pDif[i] * pDif[j] * preSum[dif+1];
			}
		}
		
		System.out.println(ans);
	}
	
	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 int[] nexIntArray() throws Throwable
		{
			st = new StringTokenizer(br.readLine());
			int[] a = new int[st.countTokens()];
			for(int i = 0; i < a.length;i++)a[i]=nextInt();
			return a;
		}
		
		public boolean ready() throws IOException {return br.ready();}


	}
}
