#include <cassert>
#include <cmath>
#include <algorithm>
#include <iostream>
#include <iomanip>
#include <climits>
#include <map>
#include <queue>
#include <set>
#include <cstring>
#include <vector>

using namespace std;
typedef long long ll;

int main() {
  int N;
  cin >> N;

  int A[N];
  for (int i = 0; i < N; ++i) {
    cin >> A[i];
  }

  int zero_cnt = 0;
  int one_cnt = 0;
  int cnt = 0;
  int div_cnt = 0;
  vector<int> swap_cnt;
  vector<int> div_size;

  for (int i = 0; i < N; ++i) {
    int a = A[i];

    if (a == 2) {
      if (cnt > 0) {
        swap_cnt.push_back(one_cnt);
        div_size.push_back(cnt);
        div_cnt++;
        cnt = 0;
        one_cnt = 0;
      }
    } else {
      cnt++;
      if (a == 1) {
        one_cnt++;
      } else {
        zero_cnt++;
      }
    }
  }

  if (cnt > 0) {
    div_cnt++;
    swap_cnt.push_back(one_cnt);
    div_size.push_back(cnt);
  }

  vector<vector<int>> dp(div_cnt + 1, vector<int>(N + 1, INT_MAX));
  dp[0][0] = 0;
  int ans = INT_MAX;

  for (int i = 1; i <= div_cnt; ++i) {
    int size = div_size[i - 1];
    int sc = swap_cnt[i - 1];

    // fprintf(stderr, "i: %d, size: %d, sc: %d\n", i, size, sc);

    for (int len = 0; len < zero_cnt; ++len) {
      if (dp[i - 1][len] == INT_MAX) continue;

      int n_size = len + size;
      int n_sc = dp[i - 1][len] + sc;

      dp[i][n_size] = min(dp[i][n_size], n_sc);
    }

    ans = min(ans, dp[i][zero_cnt]);
  }

  if (ans == INT_MAX) {
    cout << -1 << endl;
  } else {
    cout << ans << endl;
  }

  return 0;
}