import std.conv, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(T)(in T[] as, in bool delegate(T) test) { int low = -1, upp = cast(int)(as.length); for (; low + 1 < upp; ) { int mid = (low + upp) >> 1; (test(as[mid]) ? low : upp) = mid; } return upp; }
int lowerBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a < val)); }
int upperBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a <= val)); }


int N;
int[] A;

void main() {
  try {
    for (; ; ) {
      N = readInt();
      A = new int[N];
      foreach (i; 0 .. N) {
        A[i] = readInt();
      }
      
      auto dp = new real[][][](N + 1, N + 1, N + 1);
      dp[0][0][0] = 0.0L;
      foreach (a0; 0 .. N + 1)
      foreach (a1; 0 .. N + 1)
      foreach (a2; 0 .. N + 1)
      {
        if (0 < a0 + a1 + a2 && a0 + a1 + a2 <= N) {
          const p0 = 1.0L * a0 / N;
          const p1 = 1.0L * a1 / N;
          const p2 = 1.0L * a2 / N;
          real tmp = 1.0L;
          if (a0 > 0) tmp += p0 * dp[a0 - 1][a1 + 1][a2];
          if (a1 > 0) tmp += p1 * dp[a0][a1 - 1][a2 + 1];
          if (a2 > 0) tmp += p2 * dp[a0][a1][a2 - 1];
          dp[a0][a1][a2] = tmp / (p0 + p1 + p2);
        }
      }
      const ans = dp[A.count(0)][A.count(1)][A.count(2)];
      writefln("%.10f", ans);
    }
  } catch (EOFException e) {
  }
}