#include <algorithm>
#include <cmath>
#include <iomanip>
#include <iostream>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <vector>

#define mkp make_pair
#define mkt make_tuple
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define all(v) v.begin(), v.end()
using namespace std;
typedef long long ll;
const ll MOD = 1e9 + 7;
// const ll MOD = 998244353;
template <class T>
void chmin(T &a, const T &b) {
    if (a > b) a = b;
}
template <class T>
void chmax(T &a, const T &b) {
    if (a < b) a = b;
}

const int INF = 1e9;

void solve() {
    int N;
    cin >> N;
    vector<int> A(N);
    rep(i, N) cin >> A[i];

    vector<pair<int, int>> B;
    vector<int> cnt(2, 0);
    rep(i, N) {
        if (A[i] == 2) {
            if (cnt[0] + cnt[1] > 0) {
                B.push_back(mkp(cnt[0], cnt[1]));
                cnt[0] = 0;
                cnt[1] = 0;
            }
        } else {
            cnt[A[i]]++;
        }
    }
    if (cnt[0] + cnt[1] > 0) {
        B.push_back(mkp(cnt[0], cnt[1]));
        cnt[0] = 0;
        cnt[1] = 0;
    }

    int V = B.size();
    vector<int> dp(N + 1, INF);
    dp[0] = 0;
    rep(i, V) {
        auto [cz, co] = B[i];
        int cnt = cz + co;
        vector<int> ndp(N + 1, INF);
        for (int j = 0; j <= N; j++) {
            if (dp[j] == INF) continue;

            chmin(ndp[j], dp[j]);
            if (j + cnt <= N) chmin(ndp[j + cnt], dp[j] + co);
        }
        swap(dp, ndp);
    }

    int zero = 0, one = 0;
    rep(i, N) {
        if (A[i] == 0)
            zero++;
        else if (A[i] == 1)
            one++;
    }

    if (dp[zero] != INF) {
        cout << dp[zero] << endl;
    } else {
        cout << -1 << endl;
    }
}

int main() {
    cin.tie(nullptr);
    ios::sync_with_stdio(false);

    solve();

    return 0;
}