#include <bits/stdc++.h>
using namespace std;

struct iofast_t {
    iofast_t() {
        ios::sync_with_stdio(false);
        cin.tie(nullptr);
    }
} iofast;

struct uns_t {} uns;
template <typename Element, typename Head, typename ...Args>
auto vec(Element init, Head arg, Args ...args) {
    if constexpr (sizeof...(Args) == 0) return std::vector(arg, init);
    else return std::vector(arg, vec(init, args...));
}
template <typename Element, typename Head, typename ...Args>
auto vec(uns_t, Head arg, Args ...args) {
    return vec(Element(), arg, args...);
}

template <typename T, typename Compare = less<T>>
T &chmin(T &l, T r, Compare &&f = less<T>()) { return l = min(l, r, f); }
template <typename T, typename Compare = less<T>>
T &chmax(T &l, T r, Compare &&f = less<T>()) { return l = max(l, r, f); }

int main() {
    constexpr int inf = INT_MAX / 2;

    int n; cin >> n;
    auto a = vec<int>(uns, n);
    for (auto &e : a) cin >> e;

    auto b = [&] {
        auto b = vec<tuple<int, int>>(uns, 0);

        int last = 0;
        for (int i = 0; i < n; ++i) {
            if (a[i] != 2) {
                continue;
            }

            if (0 < i - last) {
                b.push_back({ i - last, count(begin(a) + last, begin(a) + i, 1) });
            }
            last = i + 1;
        }

        if (0 < n - last) {
            b.push_back({ n - last, count(begin(a) + last, begin(a) + n, 1) });
        }

        return b;
    }();

    auto dp = vec<int>(inf, size(b) + 1, n + 1);

    dp[0][0] = 0;
    for (int i = 1; i <= size(b); ++i) {
        auto [l, c] = b[i - 1];
        for (int j = 0; j <= n; ++j) {
            dp[i][j] = dp[i - 1][j];
            if (0 <= j - l) {
                chmin(dp[i][j], dp[i - 1][j - l] + c);
            }
        }
    }

    auto ans = dp[size(b)][count(begin(a), end(a), 0)];

    if (ans != inf) {
        cout << ans << endl;
    }
    else {
        cout << -1 << endl;
    }
}