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

enum inf = 1_001_001_001;
enum infl = 1_001_001_001_001_001_001L;
enum mod = 1_000_000_007L;


void main() {
    int N;
    scan(N);
    auto a = readln.split.to!(int[]);
    auto c = new int[](4);
    foreach (ai ; a) {
        c[ai]++;
    }

    auto dp = new double[][][](N + 1, N + 1, N + 1);
    fillAll(dp, -1);

    double rec(int i, int j, int k) {
        if (i + j + k == 0) {
            return 0;
        }
        if (dp[i][j][k] != -1) {
            return dp[i][j][k];
        }
        dp[i][j][k] = 0;
        if (i > 0) dp[i][j][k] += i * rec(i - 1, j, k) / (i + j + k);
        if (j > 0) dp[i][j][k] += j * rec(i + 1, j - 1, k) / (i + j + k);
        if (k > 0) dp[i][j][k] += k * rec(i, j + 1, k - 1) / (i + j + k);
        dp[i][j][k] += 1.0 * N / (i + j + k);
        return dp[i][j][k];
    }

    auto ans = rec(c[1], c[2], c[3]);
    writefln("%.12f", ans);
}







void scan(T...)(ref T args) {
    import std.stdio : readln;
    import std.algorithm : splitter;
    import std.conv : to;
    import std.range.primitives;

    auto line = readln().splitter();
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}

void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}

bool chmin(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x > arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}

bool chmax(T, U...)(ref T x, U args) {
    bool isChanged;

    foreach (arg; args) {
        if (x < arg) {
            x = arg;
            isChanged = true;
        }
    }

    return isChanged;
}
