#include<iostream>
#include<vector>
#include<unordered_set>
#include<unordered_map>
#include<cmath>
#include <algorithm>
#include <random>
using namespace std;
using ll = long long;

#define REP(i, n) for(ll i=0; i<(n);i++)
#define VL vector<ll>

std::random_device seed_gen;
std::default_random_engine eg(seed_gen());


int main() {
  ll n;
  cin >> n;
  VL a(n);
  REP(i, n)
    cin >> a[i];
  VL z, o;
  ll zc = 0, oc = 0;
  z.push_back(0);
  o.push_back(0);
  REP(i, n) {
    if (a[i] == 2) {
      z.push_back(0);
      o.push_back(0);
    }
    else if (a[i]) {
      o.back()++;
      oc++;
    }
    else {
      z.back()++;
      zc++;
    }
  }
  vector dp(z.size() + 1, vector<ll>(zc + oc + 1, 1e18));
  dp[0][zc] = 0;
  REP(i, z.size()) {
    REP(j, zc+oc+1) {
      if (j + o[i] < zc + oc + 1) {        
        dp[i + 1][j + o[i]] = min(dp[i + 1][j + o[i]], dp[i][j] + z[i]);
      }
      if (j - z[i] >= 0) {
        dp[i + 1][j - z[i]] = min(dp[i + 1][j - z[i]], dp[i][j] + o[i]);
      }
    }
  }
  if (dp[z.size()][oc] > 1e17) {
    cout << -1;
  }
  else {
    cout << dp[z.size()][oc] / 2;
  }

  return 0;
}