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

int main(){
  int n;
  cin >> n;
  vector<int>a(n);
  int cnt = 0;
  for(int i = 0; i < n;i++){
    cin >> a[i];
    if(a[i] == 0)cnt++;
  }
  const int inf = 1e9;
  vector<int>dp(cnt + 1, inf);
  dp[0] = 0;
  int cost = 0, num = 0;
  a.push_back(2);
  for(int i = 0; i <= n; i++){
    if(a[i] == 2){
      for(int j = cnt; j >= 0; j--){
        if(j + num <= cnt){
          dp[j + num] = min(dp[j + num], dp[j] + cost);
        }
      }
      cost = 0;
      num = 0;
    }
    else{
      num++;
      cost += a[i];
    }
  }
  if(dp[cnt] == inf)cout << -1 << endl;
  else cout << dp[cnt] << endl;
}