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;

void scan(T...)(ref T args) {
    auto line = readln.split;
    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;
}

enum mod = 1_000_000_007L;

struct Mint {
    long x;

    this(long a) {
        x = a % mod;
        if (x < 0) x += mod;
    }

    this(Mint a) {
        x = a.x;
    }

    ref Mint opAssign(long a) {
        this = Mint(a);
        return this;
    }

    ref Mint opOpAssign(string op)(Mint rhs) {
        static if (op == "+") {
            x += rhs.x;
            if (x >= mod) x -= mod;
        }
        static if (op == "-") {
            x -= rhs.x;
            if (x < 0) x += mod;
        }
        static if (op == "*") {
            (x *= rhs.x) %= mod;
        }
        static if (op == "/") {
            this *= rhs.inv();
        }
        return this;
    }

    ref Mint opOpAssign(string op)(long rhs) {
        static if (op == "^^") {
            this = powmod(this, rhs);
            return this;
        }
        else {
            return mixin("this " ~ op ~ "= Mint(rhs)");
        }
    }

    const Mint powmod(Mint a, long b) {
        Mint res = 1, p = a;
        while (b > 0) {
            if (b & 1) res *= p;
            p *= p;
            b /= 2;
        }
        return res;
    }

    const Mint inv() {
        return powmod(this, mod - 2);
    }

    Mint opUnary(string op)() if (s == "-") {
        return Mint(-x);
    }

    Mint opBinary(string op, T)(const T rhs) {
        return mixin("Mint(this) " ~ op ~ "= rhs");
    }

    Mint opBinaryRight(string op)(const long rhs) {
        return mixin("Mint(rhs) " ~ op ~ "= this");
    }

    bool opEquals(Mint a, Mint b) {
        return a.x == b.x;
    }

    bool opEquals(long rhs) {
        long y = rhs % mod;
        if (y < 0) y += mod;
        return x == y;
    }

    string toString() {
        import std.conv : to;
        return x.to!string;
    }
}

unittest {
    long powmod(long a, long b) {
        return b > 0 ? powmod(a, b / 2)^^2 % mod * a^^(b & 1) % mod : 1L;
    }
    auto a = Mint(2), b = Mint(3);
    assert(a + b == 5);
    assert(a + 5 == 7);
    assert(1 + b == 4);
    assert(a - b == mod - 1);
    assert(a * b == 6);
    assert(a / b == 2 * powmod(3, mod - 2));
    assert(a^^10 == 1024);

    Mint z;
    assert(z == 0);

    (z += 2) *= 3;
    assert(z == 6);
}



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


void main() {
    int N;
    scan(N);
    auto a = readln.split.to!(int[]);
    auto dp = new double[][][](N + 1, N + 1, N + 1);
    fillAll(dp, -1);
    dp[0][0][0] = 0;

    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] = 1;
        if (i) dp[i][j][k] += rec(i - 1, j, k) * i / N;
        if (j) dp[i][j][k] += rec(i + 1, j - 1, k) * j / N;
        if (k) dp[i][j][k] += rec(i, j + 1, k - 1) * k / N;
        dp[i][j][k] /= 1.0 * (i + j + k) / N;
        return dp[i][j][k];
    }

    int o, t, th;
    foreach (i ; 0 .. N) {
        if (a[i] == 1) o++;
        else if (a[i] == 2) t++;
        else if (a[i] == 3) th++;
    }
    auto ans = rec(o, t, th);

    debug {
        writeln(dp);
    }
    writefln("%.12f", ans);
}
