#include<bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using mint=modint998244353; //1000000007;
using ll=long long;
using pp=pair<int,int>;
#define sr string 
#define vc vector
#define fi first
#define se second
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define pb push_back
#define all(v) v.begin(),v.end()
#define pque priority_queue
#define bpc(a) __builtin_popcount(a)
int main(){
  int n;cin>>n;
  vc<pp>v; v.pb({0,0});
  rep(i,n){
    int a;cin>>a;
    if(a==2)v.pb({0,0});
    if(a==0)v.back().fi++;
    if(a==1)v.back().se++;
  }
  int s=0,c=0;
  n=v.size();
  rep(i,n)s+=v[i].se+v[i].fi,c+=v[i].fi;
  vc<int>dp(c+1,1e8); dp[0]=0;
  rep(i,n){
    int t=v[i].fi+v[i].se;
    auto pre=dp;
    //rep(j,c+1)dp[j]=pre[j]+v[i].fi;
    rep(j,c+1)if(j+t<=c)dp[j+t]=min(dp[j+t],pre[j]+v[i].se);
  }
  if(dp[c]==1e8)cout<<-1;
  else cout<<dp[c];
}