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

int main()
{
    int n;
    cin >> n;
    vector<int> a(n);
    for (auto &&x : a)
    {
        cin >> x;
    }
    a.push_back(2);
    vector<int> len, s;
    int k = 0, t = 0;
    for (auto &&x : a)
    {
        if (x == 2)
        {
            len.push_back(k);
            s.push_back(t);
            k = 0;
            t = 0;
        }
        else
        {
            k++;
            t += x;
        }
    }
    int m = len.size(), total = count(a.begin(), a.end(), 1);
    constexpr int inf = 1e9;
    vector dp(m + 1, vector<int>(total + 1, inf));
    dp.at(0).at(0) = 0;
    for (int i = 0; i < m; i++)
    {
        for (int j = 0; j <= total; j++)
        {
            dp.at(i + 1).at(j) = min(dp.at(i + 1).at(j), dp.at(i).at(j) + s.at(i));
            if (j + len.at(i) <= total)
            {
                dp.at(i + 1).at(j + len.at(i)) = min(dp.at(i + 1).at(j + len.at(i)), dp.at(i).at(j) + len.at(i) - s.at(i));
            }
        }
    }
    if (dp.at(m).at(total) == inf)
    {
        cout << -1 << endl;
    }
    else
    {
        cout << dp.at(m).at(total) / 2 << endl;
    }
    return 0;
}