#include "bits/stdc++.h"
//#include "atcoder/all"
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
const int INF = 1e9;
//const long long LINF = 1e18;
//const bool debug = false;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
#define P pair<int,int>
template<typename A, typename B> inline bool chmax(A & a, const B & b) { if (a < b) { a = b; return true; } return false; }
template<typename A, typename B> inline bool chmin(A & a, const B & b) { if (a > b) { a = b; return true; } return false; }

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n; cin >> n;
	vector<int>a(n);
	rep(i, n)cin >> a[i];
	a.push_back(2);
	int w = 0;
	int h = 0;
	vector<P>vp;
	rep(i, n + 1) {
		if (0 == a[i])w++;
		else if (1 == a[i])h++;
		else {
			if (w + h > 0)vp.push_back({ w,h });
			w = 0;
			h = 0;
		}
	}
	int sw = 0, sh = 0;
	rep(i, vp.size()) {
		sw += vp[i].first;
		sh += vp[i].second;
	}
	vector<vector<int>> dp(vp.size() + 1, vector<int>(sw + 1, INF));
	dp[0][0] = 0;
	rep(i, vp.size()) {
		rep(j, sw + 1) {
			if (INF == dp[i][j])continue;
			if (j + vp[i].first + vp[i].second <= sw) {
				chmin(dp[i + 1][j + vp[i].first + vp[i].second], dp[i][j] + vp[i].second);
			}
			chmin(dp[i + 1][j], dp[i][j]);
		}
	}
	if (INF == dp[vp.size()][sw])dp[vp.size()][sw] = -1;
	cout << dp[vp.size()][sw] << endl;
	return 0;
}