#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <tuple>
#include <cmath>
#include <numeric>
#include <functional>
#include <cassert>

#define debug_value(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << #x << "=" << x << endl;
#define debug(x) cerr << "line" << __LINE__ << ":<" << __func__ << ">:" << x << endl;

template<class T> inline bool chmax(T& a, T b) { if (a < b) { a = b; return 1; } return 0; }
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }

using namespace std;
typedef long long ll;


template <typename T>
class OffsetVec{
    public:
	int n;
    vector<T> v;
	T& operator[](int x) {
        return v[x+n];
	}
    void print(){
        for(int i = -n; i <= n; i++) cout << v[i+n] << ' ';
        cout << endl;
    }
    OffsetVec(int _n){
        n = _n;
        v = vector<T>(2*n+1);
    }
    OffsetVec(int _n, T x){
        n = _n;
        v = vector<T>(2*n+1, x);
    }
};

const int INF = 1e9;

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    int n; cin >> n;
    vector<int> a(n);
    for(int i = 0; i < n; i++) cin >> a[i];
    vector<int> s, t;
    vector<int> cnt(2);
    auto add = [&](){
        if(cnt[0]+cnt[1] > 0){
            s.push_back(cnt[0]);
            t.push_back(cnt[1]);
        }
    };
    for(int i = 0; i < n; i++){
        if(a[i] == 2){
            add();
            cnt[0] = 0;      
            cnt[1] = 0;      
        }else{
            cnt[a[i]]++;
        }
    }
    add();
    int m = s.size();
    vector<OffsetVec<int>> dp(m+1, OffsetVec<int>(n+1, INF));
    dp[0][0] = 0;
    for(int i = 0; i < m; i++){
        // cout << s[i] << ' ' << t[i] << endl;
        for(int j = -n; j <= n; j++){
            if(dp[i][j] == INF) continue;
            // 0の場所にする
            if(j < 0){
                chmin(dp[i+1][j+t[i]], dp[i][j]+min(-j, t[i]));
            }else{
                chmin(dp[i+1][j+t[i]], dp[i][j]);
            }
            // 1の場所にする
            if(j < 0){
                chmin(dp[i+1][j-s[i]], dp[i][j]);
            }else{
                chmin(dp[i+1][j-s[i]], dp[i][j]+min(j, s[i]));
            }
        }
    }
    if(dp[m][0] == INF) cout << -1 << endl;
    else cout << dp[m][0] << endl;
}