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

#define _P(...) (void)printf(__VA_ARGS__)
#define FOR(x,to) for(x=0;x<(to);x++)
#define FORR(x,arr) for(auto& x:arr)
#define FORR2(x,y,arr) for(auto& [x,y]:arr)
#define ALL(a) (a.begin()),(a.end())
#define ZERO(a) memset(a,0,sizeof(a))
#define MINUS(a) memset(a,0xff,sizeof(a))
template<class T> bool chmax(T &a, const T &b) { if(a<b){a=b;return 1;}return 0;}
template<class T> bool chmin(T &a, const T &b) { if(a>b){a=b;return 1;}return 0;}
//-------------------------------------------------------

int N,C;
vector<pair<int,int>> V;

int from[5050];
int to[5050];

void solve() {
	int i,j,k,l,r,x,y; string s;
	
	cin>>N;
	V.push_back({0,0});
	FOR(i,N) {
		cin>>x;
		if(x==2) {
			if(V.back().first+V.back().second) V.push_back({0,0});
		}
		else if(x==0) {
			C++;
			V.back().first++;
		}
		else if(x==1) {
			V.back().second++;
		}
	}
	
	
	FOR(i,N+1) from[i]=1<<30;
	from[0]=0;
	FORR(v,V) {
		FOR(i,N+1) to[i]=1<<30;
		FOR(i,N+1) if(from[i]<1<<30) {
			to[i+v.first+v.second]=min(to[i+v.first],from[i]+v.second);
			to[i]=min(to[i],from[i]);
		}
		swap(from,to);
	}
	if(from[C]>=1<<30) cout<<-1<<endl;
	else cout<<from[C]<<endl;
}


int main(int argc,char** argv){
	string s;int i;
	if(argc==1) ios::sync_with_stdio(false), cin.tie(0);
	FOR(i,argc-1) s+=argv[i+1],s+='\n'; FOR(i,s.size()) ungetc(s[s.size()-1-i],stdin);
	cout.tie(0); solve(); return 0;
}