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

int main() {
    int n;
    cin >> n;

    vector<int> a(n);
    int s = 0;
    for (int i = 0; i < n; i++) {
        cin >> a[i];
        s += a[i] == 0;
    }
    a.push_back(2);

    vector<array<int, 2>> p;
    array<int, 2> t = {};
    for (int i = 0; i <= n; i++) {
        if (a[i] == 2) {
            if (t[0] + t[1] > 0) {
                p.push_back(t);
                t = {};
            }
        } else {
            t[a[i]]++;
        }
    }

    int dp[5001];//0の個数がkのときの最小コスト
    for (int k = 0; k <= s; k++) {
        dp[k] = 1 << 20;
    }
    dp[0] = 0;
    for (const auto &t : p) {
        for (int k = s; k >= 0; k--) {
            dp[k] += t[0];
            if (int k1 = k - (t[0] + t[1]); k1 >= 0) dp[k] = min(dp[k], dp[k1] + t[1]);
        }
    }
    int r = dp[s] / 2;
    if (dp[s] >= 1 << 20) r = -1;
    cout << r << endl;

    return 0;
}