#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
using namespace std;
#define FOR(i,m,n) for(int i=(m);i<(n);++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(),(v).end()
using ll = long long;
constexpr int INF = 0x3f3f3f3f;
constexpr long long LINF = 0x3f3f3f3f3f3f3f3fLL;
constexpr double EPS = 1e-8;
constexpr int MOD = 1000000007;
// constexpr int MOD = 998244353;
constexpr int dy[] = {1, 0, -1, 0}, dx[] = {0, -1, 0, 1};
constexpr int dy8[] = {1, 1, 0, -1, -1, -1, 0, 1}, dx8[] = {0, -1, -1, -1, 0, 1, 1, 1};
template <typename T, typename U> inline bool chmax(T &a, U b) { return a < b ? (a = b, true) : false; }
template <typename T, typename U> inline bool chmin(T &a, U b) { return a > b ? (a = b, true) : false; }
struct IOSetup {
  IOSetup() {
    std::cin.tie(nullptr);
    std::ios_base::sync_with_stdio(false);
    std::cout << fixed << setprecision(20);
  }
} iosetup;

int main() {
  int n; cin >> n;
  vector<int> a(n); REP(i, n) cin >> a[i];
  vector<pair<int, int>> b;
  int cnt[2]{};
  REP(i, n) {
    if (a[i] == 2) {
      if (cnt[0] > 0 || cnt[1] > 0) b.emplace_back(cnt[0], cnt[1]);
      cnt[0] = cnt[1] = 0;
    } else {
      ++cnt[a[i]];
    }
  }
  if (cnt[0] > 0 || cnt[1] > 0) b.emplace_back(cnt[0], cnt[1]);
  vector<int> dp(n * 2 + 1, INF);
  dp[n] = 0;
  for (auto [x, y] : b) {
    vector<int> nx(n * 2 + 1, INF);
    REP(i, n * 2 + 1) {
      if (i + y <= n * 2) chmin(nx[i + y], dp[i] + y);
      if (i - x >= 0) chmin(nx[i - x], dp[i]);
    }
    dp.swap(nx);
  }
  cout << (dp[n] == INF ? -1 : dp[n]) << '\n';
  return 0;
}