#include <iostream>
#include <vector>
#include <algorithm>
#include <cmath>
#include <queue>
#include <string>
#include <map>
#include <set>
#include <stack>
#include <tuple>
#include <deque>
#include <array>
#include <numeric>
#include <bitset>
#include <iomanip>
#include <cassert>
#include <chrono>
#include <random>
#include <limits>
#include <iterator>
#include <functional>
#include <sstream>
#include <fstream>
#include <complex>
#include <cstring>
#include <unordered_map>
#include <unordered_set>
using namespace std;

// #pragma GCC optimize("O3")
// #pragma GCC optimize("unroll-loops")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,tune=native")
// #pragma GCC target("avx512f,avx512dq,avx512cd,avx512bw,avx512vl")

using ll = long long;
constexpr int INF = 1001001001;
// constexpr int mod = 1000000007;
constexpr int mod = 998244353;

template<class T>
inline bool chmax(T& x, T y){
    if(x < y){
        x = y;
        return true;
    }
    return false;
}
template<class T>
inline bool chmin(T& x, T y){
    if(x > y){
        x = y;
        return true;
    }
    return false;
}

int dp[5001][5001][3];

int main(){
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N, zero = 0;
    cin >> N;
    vector<int> A(N);
    for(int i = 0; i < N; ++i){
        cin >> A[i];
        zero += A[i] == 0;
    }

    for(int i = 0; i <= N; ++i){
        for(int j = 0; j <= N; ++j){
            for(int k = 0; k <= 2; ++k){
                dp[i][j][k] = INF;
            }
        }
    }

    dp[0][0][2] = 0;
    for(int i = 0; i < N; ++i){
        if(A[i] == 2){
            for(int j = 0; j <= i; ++j){
                for(int k = 0; k <= 2; ++k){
                    chmin(dp[i + 1][j][2], dp[i][j][k]);
                }
            }
        }
        else{
            for(int j = 0; j <= i; ++j){
                chmin(dp[i + 1][j + 1][0], dp[i][j][0] + (A[i] == 1));
                chmin(dp[i + 1][j][1], dp[i][j][1] + (A[i] == 0));
                chmin(dp[i + 1][j + 1][0], dp[i][j][2] + (A[i] == 1));
                chmin(dp[i + 1][j][1], dp[i][j][2] + (A[i] == 0));
            }
        }
    }

    int ans = dp[N][zero][A[N - 1]];
    if(ans == INF)  ans = -1;
    else    ans /= 2;

    cout << ans << endl;

    return 0;
}