// ---------- begin chmin, chmax ----------
pub trait ChangeMinMax {
    fn chmin(&mut self, x: Self) -> bool;
    fn chmax(&mut self, x: Self) -> bool;
}

impl<T: PartialOrd> ChangeMinMax for T {
    fn chmin(&mut self, x: Self) -> bool {
        *self > x && {
            *self = x;
            true
        }
    }
    fn chmax(&mut self, x: Self) -> bool {
        *self < x && {
            *self = x;
            true
        }
    }
}
// ---------- end chmin, chmax ----------
// ---------- begin input macro ----------
// reference: https://qiita.com/tanakh/items/0ba42c7ca36cd29d0ac8
macro_rules! input {
    (source = $s:expr, $($r:tt)*) => {
        let mut iter = $s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
    ($($r:tt)*) => {
        let s = {
            use std::io::Read;
            let mut s = String::new();
            std::io::stdin().read_to_string(&mut s).unwrap();
            s
        };
        let mut iter = s.split_whitespace();
        input_inner!{iter, $($r)*}
    };
}

macro_rules! input_inner {
    ($iter:expr) => {};
    ($iter:expr, ) => {};
    ($iter:expr, $var:ident : $t:tt $($r:tt)*) => {
        let $var = read_value!($iter, $t);
        input_inner!{$iter $($r)*}
    };
}

macro_rules! read_value {
    ($iter:expr, ( $($t:tt),* )) => {
        ( $(read_value!($iter, $t)),* )
    };
    ($iter:expr, [ $t:tt ; $len:expr ]) => {
        (0..$len).map(|_| read_value!($iter, $t)).collect::<Vec<_>>()
    };
    ($iter:expr, chars) => {
        read_value!($iter, String).chars().collect::<Vec<char>>()
    };
    ($iter:expr, bytes) => {
        read_value!($iter, String).bytes().collect::<Vec<u8>>()
    };
    ($iter:expr, usize1) => {
        read_value!($iter, usize) - 1
    };
    ($iter:expr, $t:ty) => {
        $iter.next().unwrap().parse::<$t>().expect("Parse error")
    };
}
// ---------- end input macro ----------

use std::collections::*;
use std::io::Write;

type Map<K, V> = BTreeMap<K, V>;
type Set<T> = BTreeSet<T>;
type Deque<T> = VecDeque<T>;

fn run() {
    input! {
        n: usize,
        a: [u8; n],
    }
    let mut a = a;
    a.push(2);
    let inf = 2 * n + 1;
    let mut dp = Map::new();
    dp.insert((0, 0), 0);
    let mut x = 0;
    let mut y = 0;
    let mut sum = [0, 0, 0];
    for a in a.iter() {
        sum[*a as usize] += 1;
        if *a == 2 {
            if (x, y) == (0, 0) {
                continue;
            }
            let len = x + y;
            let mut next = Map::new();
            for (a, cost) in dp {
                next.entry((a.0 + len, a.1)).or_insert(inf).chmin(cost + x);
                next.entry((a.0, a.1 + len)).or_insert(inf).chmin(cost + y);
            }
            dp = next;
            x = 0;
            y = 0;
        } else if *a == 1 {
            x += 1;
        } else {
            y += 1;
        }
    }
    let mut ans = inf;
    for ((a, b), c) in dp {
        if (a, b) == (sum[0], sum[1]) {
            ans.chmin(c);
        }
    }
    if ans < inf {
        println!("{}", ans / 2);
    } else {
        println!("-1");
    }
}

fn main() {
    run();
}