#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001


int main(){
	
	int N;
	cin>>N;
	
	vector<int> A(N);
	rep(i,N){
		cin>>A[i];
	}
	
	vector<int> l,r;
	rep(i,N){
		l.push_back(i);
		for(int j=i;j<N;j++){
			if(A[j]==2){
				r.push_back(j);
				break;
			}
		}
		if(l.size()!=r.size()){
			r.push_back(N);
		}
		i = r.back();
	}
	
	int cnt = 0;
	rep(i,N){
		if(A[i]==1){
			cnt++;
		}
	}
	
	vector<int> cost(l.size(),0);
	
	rep(i,l.size()){
		for(int j=l[i];j<r[i];j++){
			if(A[j]==0){
				cost[i]++;
			}
		}
	}
	
	vector dp(N+1,Inf);
	dp[0] = 0;
	
	rep(i,l.size()){
		vector ndp(N+1,Inf);
		rep(j,N+1){
			ndp[j] = min(ndp[j],dp[j]);
			int nj = j + (r[i] - l[i]);
			if(nj>N)continue;
			ndp[nj] = min(ndp[nj],dp[j] + cost[i]);
		}
		swap(dp,ndp);
	}
	
	int ans = dp[cnt];
	if(ans==Inf)ans = -1;
	
	cout<<ans<<endl;
	
	return 0;
}