macro_rules! debug {
    ( $x:expr ) => {
        if cfg!(debug_assertions) {
            println!("{}:{} {} = {:?}", line!(), column!(), stringify!($x), $x);
        }
    }
}


fn main() {
    use std::collections::HashMap;

    let stdin = std::io::stdin();
    let mut stdin = my::io::InputReader::new(stdin.lock());

    let N = stdin.read();
    let mut a = vec![0; N];
    for i in 0..N {
        a[i] = stdin.read();
    }

    type Triple = (usize, usize, usize);

    struct E {
        N: usize,
        m: Vec<Vec<Vec<f64>>>,
    }

    impl E {
        fn new(N: usize) -> E {
            let mut m = vec![vec![vec![-1f64; N + 1]; N + 1]; N + 1];
            m[0][0][0] = 0f64;
            E {
                N: N,
                m: m,
            }
        }

        fn calc(&mut self, t: Triple) -> f64 {
            debug!(t);
            let (c1, c2, c3) = t;
            if self.m[c1][c2][c3] != -1f64 {
                return self.m[c1][c2][c3];
            }
            let c0 = self.N - c1 - c2 - c3;
            let p0 = (c0 as f64) / (self.N as f64);
            let p1 = (c1 as f64) / (self.N as f64);
            let p2 = (c2 as f64) / (self.N as f64);
            let p3 = (c3 as f64) / (self.N as f64);
            let mut x = 1f64;
            if c1 > 0 { x += p1 * self.calc((c1 - 1, c2, c3)); }
            if c2 > 0 { x += p2 * self.calc((c1 + 1, c2 - 1, c3)); }
            if c3 > 0 { x += p3 * self.calc((c1, c2 + 1, c3 - 1)); }
            let result = x / (1f64 - p0);
            self.m[c1][c2][c3] = result;
            result
        }

    }

    fn count<T: Eq>(a: &[T], x: T) -> usize {
        a.iter().filter(|&y| *y == x).count()
    }

    let mut x = E::new(N);
    let c1 = count(&a, 1);
    let c2 = count(&a, 2);
    let c3 = count(&a, 3);
    debug!(c1);
    debug!(c2);
    debug!(c3);
    println!("{}", x.calc((c1, c2, c3)));
}

mod my {
    pub mod io {
        use std::io::{BufRead, ErrorKind, Result};
        use std::str::{FromStr, from_utf8};
        use std::fmt::Debug;

        pub fn read_word<R: BufRead>(r: &mut R, buf: &mut Vec<u8>)
                                              -> Result<usize> {
            let mut read = 0;
            loop {
                let (done, used) = {
                    let available = match r.fill_buf() {
                        Ok(n) => n,
                        Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                        Err(e) => return Err(e)
                    };
                    match available.iter().position(|&x| x == b'\n' || x == b' ') {
                        Some(i) => {
                            buf.extend_from_slice(&available[..i]);
                            (true, i + 1)
                        }
                        None => {
                            buf.extend_from_slice(available);
                            (false, available.len())
                        }
                    }
                };
                r.consume(used);
                read += used;
                if done || used == 0 {
                    return Ok(read);
                }
            }
        }

        pub struct InputReader<R: BufRead> {
            reader: R,
            buf: Vec<u8>,
        }

        impl<R: BufRead> InputReader<R> {
            pub fn new(reader: R) -> InputReader<R> {
                InputReader {
                    reader: reader,
                    buf: Vec::<u8>::new(),
                }
            }

            pub fn read_word(&mut self) {
                read_word(&mut self.reader, &mut self.buf);
            }
            
            pub fn read<T>(&mut self) -> T
                where T: FromStr,
                      T::Err: Debug
            {
                self.buf.clear();
                self.read_word();
                from_utf8(&self.buf).unwrap().trim().parse().unwrap()
            }
        }
    }
}


Measured memory_bytes: 237948