#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;

vector<pair<int, int> > v;
int d[5009][5009], inf = 1000000000;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n; cin >> n;
	int m = 0, c0 = 0, c1 = 0;
	for (int i = 0; i <= n; i++) {
		int a = 2;
		if (i < n) cin >> a;
		if (a == 0) {
			c0++; m++;
		}
		else if (a == 1) c1++;
		else if (c0 || c1) {
			v.push_back(make_pair(c0 + c1, c1));
			c0 = c1 = 0;
		}
	}
	for (int i = 1; i <= m; i++)
		d[0][i] = inf;
	for (int i = 1; i <= v.size(); i++) {
		int a = v[i - 1].first, w = v[i - 1].second;
		for (int j = 0; j <= m; j++) {
			d[i][j] = d[i - 1][j];
			if (j >= a) d[i][j] = min(d[i][j], d[i - 1][j - a] + w);
		}
	}
	int ans = d[(int)v.size()][m];
	if (ans >= inf) cout << -1 << '\n';
	else cout << ans << '\n';
	return 0;
}