#include <bits/stdc++.h>

using i32 = std::int32_t;
using u32 = std::uint32_t;
using i64 = std::int64_t;
using u64 = std::uint64_t;
using i128 = __int128_t;
using u128 = __uint128_t;
using isize = std::ptrdiff_t;
using usize = std::size_t;

class rep {
    struct Iter {
        usize itr;
        constexpr Iter(const usize pos) noexcept : itr(pos) {}
        constexpr void operator++() noexcept { ++itr; }
        constexpr bool operator!=(const Iter& other) const noexcept { return itr != other.itr; }
        constexpr usize operator*() const noexcept { return itr; }
    };
    const Iter first, last;

  public:
    explicit constexpr rep(const usize first, const usize last) noexcept : first(first), last(std::max(first, last)) {}
    constexpr Iter begin() const noexcept { return first; }
    constexpr Iter end() const noexcept { return last; }
};

template <class T, T Div = 2> constexpr T INFTY = std::numeric_limits<T>::max() / Div;

template <class T> bool setmin(T& lhs, const T& rhs) {
    if (lhs > rhs) {
        lhs = rhs;
        return true;
    }
    return false;
}

template <class T> using Vec = std::vector<T>;

void main_() {
    usize N;
    std::cin >> N;
    usize a = 0, b = 0, all = 0;
    Vec<usize> dp(N + 1, INFTY<usize>);
    dp[0] = 0;
    const auto process = [&] {
        if (a == 0 and b == 0) {
            return;
        }
        Vec<usize> next(N + 1, INFTY<usize>);
        for (const auto i : rep(0, N + 1)) {
            setmin(next[i], dp[i] + a);
        }
        for (const auto i : rep(0, N + 1)) {
            if (i + a + b <= N) {
                setmin(next[i + a + b], dp[i] + b);
            }
        }
        dp = std::move(next);
        all += a;
        a = b = 0;
    };
    for (const auto i : rep(0, N)) {
        char c;
        std::cin >> c;
        if (c == '2') {
            process();
        } else {
            (c == '0' ? a : b) += 1;
        }
    }
    process();
    if (dp[all] == INFTY<usize>) {
        std::cout << -1 << '\n';
    } else {
        std::cout << dp[all] / 2 << '\n';
    }
}

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(nullptr);
    main_();
    return 0;
}