#include <bits/stdc++.h>
using namespace std;
int main(){
    int N;
    cin >> N;
    vector<int> A(N);
    for(int i = 0; i < N; i++) cin >> A[i];
    vector<int> cnt;
    vector<int> z, o;
    int zero = 0, one = 0;
    for(int i = 0; i < N; i++){
        if(A[i] == 0) zero++;
        else if(A[i] == 1) one++;
        else{
            cnt.push_back(zero + one);
            o.push_back(one);
            z.push_back(zero);
            one = 0;
            zero = 0;
        }
    }
    if(one + zero != 0){
        cnt.push_back(zero + one);
        o.push_back(one);
        z.push_back(zero);
    }
    int sum = 0;
    zero = 0, one = 0;
    for(int i = 0; i < cnt.size(); i++){
        sum += cnt[i];
        one += o[i];
        zero += z[i];
    }
    vector<vector<int>> dp((int)cnt.size() + 1, vector<int>(sum + 1, -1));
    dp[0][0] = 0;
    for(int i = 0; i < (int)cnt.size(); i++){
        for(int j = 0; j < sum + 1; j++){
            if(dp[i][j] != -1){
                dp[i + 1][j] = max(dp[i + 1][j], dp[i][j]);
                dp[i + 1][j + cnt[i]] = max(dp[i + 1][j + cnt[i]], dp[i][j] + o[i]);
            }
        }
    }
    if(dp[cnt.size()][one] == -1) cout << -1 << endl;
    else{
        cout << one - dp[cnt.size()][one] << endl;
    }
}