#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define ALL(v) v.begin(), v.end()
typedef long long ll;

#include <bits/stdc++.h>
using namespace std;

int dp[5010][5010];
const int INF=1e9+7;

int main(){
  int n;
  cin>>n;
  
  vector<int> A(n+1);
  rep(i,n) cin>>A[i];
  A[n]=2;
  
  vector<int> B,C,D;
  int cnt0=0,cnt1=0,sum0=0;
  rep(i,n+1){
    if(A[i]==0){
      cnt0++;
      sum0++;
    }
    else if(A[i]==1) cnt1++;
    else{
      if(cnt0==0 && cnt1==0) continue;
      else{
        B.push_back(cnt0);
        C.push_back(cnt1);
        D.push_back(cnt0+cnt1);
        cnt0=0,cnt1=0;
      }
    }
  }
  
  rep(i,5010) rep(j,5010) dp[i][j]=INF;
  dp[0][sum0]=0;
  int m=B.size();
  rep(i,m){
    rep(j,sum0+1){
      if(dp[i][j]==INF) continue;
      dp[i+1][j]=min(dp[i+1][j],dp[i][j]+B[i]);
      if(j-D[i]>=0) dp[i+1][j-D[i]]=min(dp[i+1][j-D[i]],dp[i][j]+C[i]);
    }
  }
  
  if(dp[m][0]==INF) cout<<-1<<endl;
  else cout<<dp[m][0]/2<<endl;

  return 0;
}