import java.util.Scanner;


public class Main_yukicoder108 {

    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);

        int n = sc.nextInt();
        int[] cards = new int[3];
        for (int i = 0; i < n; i++) {
        	int a = sc.nextInt();
        	if (a >= 3) {
        		cards[2]++;
        		cards[1]++;
        		cards[0]++;
        	} else if (a == 2) {
        		cards[1]++;
        		cards[0]++;
        	} else if (a == 1) {
        		cards[0]++;
        	}
        }
        
        double[][][] dp = new double[n + 1][n + 1][n + 1];
        dp[n][n][n] = 0;
        for (int k = n - 1; k >= 0; k--) {
        	for (int j = n; j >= k; j--) {
        		for (int i = n; i >= j; i--) {
        			if (i == n && j == n) {
        				dp[i][j][k] = (dp[i][j][k + 1] * (j - k) / n + 1) * n / (j - k);
        			} else if (i == n) {
        				dp[i][j][k] = (dp[i][j + 1][k] * (i - j) / n + dp[i][j][k + 1] * (j - k) / n + 1) * n / (i - k);
        			} else {
        				dp[i][j][k] = (dp[i + 1][j][k] * (n - i) / n + dp[i][j + 1][k] * (i - j) / n + dp[i][j][k + 1] * (j - k) / n + 1) * n / (n - k);
        			}
        		}
        	}
        }

        System.out.println(dp[cards[0]][cards[1]][cards[2]]);
//        System.out.println(dp[0][0][0]);

        sc.close();
    }
}