#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#define _GLIBCXX_DEBUG
using namespace std;
using std::cout;
using std::cin;
using std::endl;
using ll=long long;
using ld=long double;
ll I=1167167167167167167;
ll mod=1e9+7;
#define rep(i,a) for (ll i=0;i<a;i++)
template<class T> using _pq = priority_queue<T, vector<T>, greater<T>>;
template<class T> ll LB(vector<T> &v,T a){return lower_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> ll UB(vector<T> &v,T a){return upper_bound(v.begin(),v.end(),a)-v.begin();}
template<class T> bool chmin(T &a,const T &b){if(a>b){a=b;return 1;}else return 0;}
template<class T> bool chmax(T &a,const T &b){if(a<b){a=b;return 1;}else return 0;}
template<class T> void So(vector<T> &v) {sort(v.begin(),v.end());}
template<class T> void Sore(vector<T> &v) {sort(v.begin(),v.end(),[](T x,T y){return x>y;});}
template<class T> void print_tate(vector<T> &v) {rep(i,v.size()) cout<<v[i]<<"\n";}
void yneos(bool a){if(a) cout<<"Yes\n"; else cout<<"No\n";}



//おちこんだりもしたけれど、私はげんきです。
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	
	int N;
	cin>>N;
	int zero=0;
	vector<int> p(N);
	rep(i,N){
		cin>>p[i];
		if(p[i]==0) zero++;
	}
	vector<int> dp(N+1,mod);
	int A=0,B=0;
	p.push_back(2);
	dp[0]=0;
	rep(i,N+1){
		if(p[i]==0) A++;
		else if(p[i]==1) B++;
		else{
			for(int j=N;j>=A+B;j--){
				chmin(dp[j],dp[j-A-B]+B);
			}
			A=0,B=0;
		}
	}
	if(dp[zero]==mod) cout<<"-1\n";
	else cout<<dp[zero]<<"\n";
	return 0;
}