#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i = 0; i < n; i++)
#define rep2(i, x, n) for(int i = x; i <= n; i++)
#define rep3(i, x, n) for(int i = x; i >= n; i--)
#define each(e, v) for(auto &e: v)
#define pb push_back
#define eb emplace_back
#define all(x) x.begin(), x.end()
#define rall(x) x.rbegin(), x.rend()
#define sz(x) (int)x.size()
using ll = long long;
using pii = pair<int, int>;
using pil = pair<int, ll>;
using pli = pair<ll, int>;
using pll = pair<ll, ll>;
const int MOD = 1000000007;
//const int MOD = 998244353;
const int inf = (1<<30)-1;
const ll INF = (1LL<<60)-1;
template<typename T> bool chmax(T &x, const T &y) {return (x < y)? (x = y, true) : false;};
template<typename T> bool chmin(T &x, const T &y) {return (x > y)? (x = y, true) : false;};

struct io_setup{
    io_setup(){
        ios_base::sync_with_stdio(false);
        cin.tie(NULL);
        cout << fixed << setprecision(15);
    }
} io_setup;

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

    vector<int> A(N);
    rep(i, N) cin >> A[i];

    vector<vector<int>> dp(N+1, vector<int>(3, inf));
    dp[0][2] = 0;
    int K = 0;

    rep(i, N){
        vector<vector<int>> tmp(N+1, vector<int>(3, inf));

        if(A[i] == 0) K++;

        rep(j, N+1){
            rep(k, 3){
                if(dp[j][k] == inf) continue;
                if(A[i] == 2){
                    chmin(tmp[j][2], dp[j][k]);
                }
                else{
                    rep(l, 2){
                        if(k != 2 && k != l) continue;
                        chmin(tmp[j+(l == 0 ? 1 : 0)][l], dp[j][k]+(l^A[i] == 1? 0 : 1));
                    }
                }
            }
        }

        swap(dp, tmp);
    }

    int ans = *min_element(all(dp[K]));

    cout << (ans == inf? -1 : ans/2) << '\n';
}