import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.Reader;
import java.util.*;

public class D {
    public static void main(String[] args) {
        FastScanner sc = new FastScanner();

        int numBalls = sc.nextInt(); // 2 <= numBalls <= 2000
        int[] balls = sc.readIntArray(numBalls);
        shuffle(balls);
        Arrays.sort(balls);
//        boolean[] hasBall = new boolean[MAX_BALL];
//        for (int a : balls) {
//            hasBall[a] = true;
//        }
//        int[] prefixSums = new int[MAX_BALL];
//        for (int i = 1; i < MAX_BALL; i++) {
//            prefixSums[i] = prefixSums[i-1];
//            if (hasBall[i]) prefixSums[i]++;
//        }
//        int[] suffixSums = new int[MAX_BALL+1];
//        for (int i = MAX_BALL-1; i >= 0; i--) {
//            suffixSums[i] = suffixSums[i+1];
//            if (hasBall[i]) suffixSums[i]++;
//        }

        long[] diffs1 = new long[MAX_BALL];
//        long[] prefixSumsDiffs1 = new long[MAX_BALL];
        List<Integer> possible1stRoundA = new ArrayList<>();
        List<Integer> possible1stRoundJ = new ArrayList<>();
        for (int i = 1; i < numBalls; i++) {
            for (int j = 0; j < i; j++) {
                possible1stRoundA.add(balls[i]);
                possible1stRoundJ.add(balls[j]);
                int diff = balls[i]-balls[j];
                diffs1[diff]++;
            }
        }
//        for (int i = 1; i < MAX_BALL; i++) {
//            prefixSumsDiffs1[i] = prefixSumsDiffs1[i-1] + diffs1[i];
//        }

        long[] diffs2 = new long[MAX_DIFF];
        for (int diff2 = 1; diff2 < MAX_BALL; diff2++) {
            long diff2Count = diffs1[diff2];
            for (int diff1 = 1; diff1 < MAX_BALL; diff1++) {
                long diff1Count = diffs1[diff1];
                int sumDiff = diff1+diff2;
                diffs2[sumDiff] += diff1Count * diff2Count;
            }
        }
        long[] prefixSumsDiffs2 = new long[MAX_DIFF];
        for (int i = 1; i < MAX_DIFF; i++) {
            prefixSumsDiffs2[i] = prefixSumsDiffs2[i-1] + diffs2[i];
//            System.out.printf("%d: %d\n", i, diffs2[i]);
        }

        List<Integer> possible3rdRoundJ = possible1stRoundA;
        List<Integer> possible3rdRoundA = possible1stRoundJ;
        double den = possible1stRoundA.size();
        den *= possible1stRoundA.size();
        den *= possible1stRoundA.size();
        double num = 0;
        for (int i = 0; i < possible3rdRoundJ.size(); i++) {
            int J3 = possible3rdRoundJ.get(i);
            int A3 = possible3rdRoundA.get(i);
            int diff = J3-A3;
            long count = prefixSumsDiffs2[diff-1];
            num += count;
        }

        double ans = num/den;
        System.out.printf("%.20f", ans);
    }

    static final int MAX_BALL = 5001;
    static final int MAX_DIFF = MAX_BALL*2;

    static void shuffle(int[] arr) {
        Random rng = new Random();
        int length = arr.length;
        for (int idx = 0; idx < arr.length; idx++) {
            int toSwap = idx + rng.nextInt(length-idx);
            int tmp = arr[idx];
            arr[idx] = arr[toSwap];
            arr[toSwap] = tmp;
        }
    }

    public static class FastScanner {
        BufferedReader br;
        StringTokenizer st;

        public FastScanner(Reader in) {
            br = new BufferedReader(in);
        }

        public FastScanner() {
            this(new InputStreamReader(System.in));
        }

        String next() {
            while (st == null || !st.hasMoreElements()) {
                try {
                    st = new StringTokenizer(br.readLine());
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            return st.nextToken();
        }

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

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

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

        // Slightly different from java.util.Scanner.nextLine(),
        // which returns any remaining characters in current line,
        // if any.
        String readNextLine() {
            String str = "";
            try {
                str = br.readLine();
            } catch (IOException e) {
                e.printStackTrace();
            }
            return str;
        }

        int[] readIntArray(int n) {
            int[] a = new int[n];
            for (int idx = 0; idx < n; idx++) {
                a[idx] = nextInt();
            }
            return a;
        }
        long[] readLongArray(int n) {
            long[] a = new long[n];
            for (int idx = 0; idx < n; idx++) {
                a[idx] = nextLong();
            }
            return a;
        }
    }
}
