#include <bits/stdc++.h>
#include <atcoder/all>
typedef long long int ll;
typedef long long int ull;
#define MP make_pair
using namespace std;
using namespace atcoder;
typedef pair<ll, ll> P;
// const ll MOD = 998244353;
const ll MOD = 1000000007;
// using mint = modint998244353;
using mint = modint1000000007;
const double pi = 3.1415926536;
const int MAX = 2000003;
long long fac[MAX], finv[MAX], inv[MAX];
void COMinit() {
    fac[0] = fac[1] = 1;
    finv[0] = finv[1] = 1;
    inv[1] = 1;
    for (int i = 2; i < MAX; i++){
        fac[i] = fac[i - 1] * i % MOD;
        inv[i] = MOD - inv[MOD%i] * (MOD / i) % MOD;
        finv[i] = finv[i - 1] * inv[i] % MOD;
    }
}
// 二項係数計算
long long COM(int n, int k){
    if (n < k) return 0;
    if (n < 0 || k < 0) return 0;
    return fac[n] * (finv[k] * finv[n - k] % MOD) % MOD;
}
ll gcd(ll x, ll y) {
   if (y == 0) return x;
   else if (y > x) {
       return gcd (y, x); 
   }
   else return gcd(x % y, y);
}
ll lcm (ll x, ll y) {
   return x / gcd(x, y) * y;
}
ll my_sqrt(ll x) {
    ll m = 0;
    ll M = 3000000001;
    while (M - m > 1) {
        ll now = (M + m) / 2;
        if (now * now <= x) {
            m = now;
        }
        else {
            M = now;
        }
    }
    return m;
}
ll keta(ll n) {
    ll ret = 0;
    while (n) {
        n /= 10;
        ret++;
    }
    return ret;
}
ll ceil(ll n, ll m) {
    // n > 0, m > 0
    ll ret = n / m;
    if (n % m) ret++;
    return ret;
}
ll pow_ll(ll x, ll n) {
    if (n == 0) return 1;
    if (n % 2) {
        return pow_ll(x, n - 1) * x;
    }
    else {
        ll tmp = pow_ll(x, n / 2);
        return tmp * tmp;
    }
}

ll dp[5001][5001];
ll INF = 100000000000;
int main() {
    int n;
    cin >> n;
    int a[n];
    for (int i = 0; i < n; i++) cin >> a[i];
    int now0 = 0;
    int now1 = 0;
    vector<P> v;
    int all_z = 0;
    for (int i = 0; i < n; i++) {
        if (a[i] == 2) {
            v.push_back(P(now0, now1));
            now0 = 0;
            now1 = 0;
        }
        else if (a[i] == 0) {
            now0++;
            all_z++;
        }
        else {
            now1++;
        }
    }
    v.push_back(P(now0, now1));
    int r = v.size();
    for (int i = 0; i <= 5000; i++) {
        for (int j = 0; j <= 5000; j++) {
            dp[i][j] = INF;
        }
    }
    dp[0][0] = 0;
    for (int i = 0; i < r; i++) {
        for (int j = 0; j <= 5000; j++) {
            // 0埋めする場合
            if (j + v[i].first + v[i].second <= 5000) {
                dp[i + 1][j + v[i].first + v[i].second] = min(dp[i + 1][j + v[i].first + v[i].second], dp[i][j] + v[i].second);
            }
            // 1埋めする場合
            dp[i + 1][j] = min(dp[i + 1][j], dp[i][j]);
        }
    }
    if (dp[r][all_z] == INF) cout << -1 << endl;
    else cout << dp[r][all_z] << endl;
    return 0;
}