#include <bits/stdc++.h>
using namespace std;
const int INF = 10000000;
int main(){
  int N;
  cin >> N;
  vector<int> A(N);
  for (int i = 0; i < N; i++){
    cin >> A[i];
  }
  vector<int> cnt0(1, 0), cnt1(1, 0);
  int s0 = 0;
  int cnt = 1;
  for (int i = 0; i < N; i++){
    if (A[i] == 0){
      cnt0.back()++;
      s0++;
    }
    if (A[i] == 1){
      cnt1.back()++;
    }
    if (A[i] == 2){
      cnt0.push_back(0);
      cnt1.push_back(0);
      cnt++;
    }
  }
  vector<vector<int>> dp(cnt + 1, vector<int>(s0 + 1, INF));
  dp[0][0] = 0;
  for (int i = 0; i < cnt; i++){
    for (int j = 0; j <= s0; j++){
      if (j + cnt0[i] + cnt1[i] <= s0){
        dp[i + 1][j + cnt0[i] + cnt1[i]] = min(dp[i + 1][j + cnt0[i] + cnt1[i]], dp[i][j] + cnt1[i]);
      }
      dp[i + 1][j] = min(dp[i + 1][j], dp[i][j]);
    }
  }
  if (dp[cnt][s0] == INF){
    cout << -1 << endl;
  } else {
    cout << dp[cnt][s0] << endl;
  }
}