//  Created On : 2026-10-06 22:13:09

#include <cassert>
#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define returnNO  {std::cout << "NO\n";  return;}
#define returnYES {std::cout << "YES\n"; return;}
#define returnAns(x) {cout << x << "\n"; return;}

template<typename T>
using MaxHeap = std::priority_queue<T, std::vector<T>, std::less<T>>;
template<typename T>
using MinHeap = std::priority_queue<T, std::vector<T>, std::greater<T>>;

const int MOD1 = 1000000007;
const int MOD2 = 998244353;


/** obs **


**/


void solve(int test_case_index) {  
    int n; cin >> n;
    vector<int> row(n); for (int& a: row) cin >> a; 

    // only build carni sum
    vector<pair<int, int>> arr; arr.reserve(n); // (len, 0s)

    int last = -1, carni = 0;
    for (int i = 0; i < n; ++i) {
        if (row[i] == 2) {
            if (carni) arr.push_back({i - last - 1, carni});
            last = i, carni = 0;
        }
        else if (row[i] == 0) carni += 1;
    }
    if (carni) arr.push_back({n - 1 - last, carni});

    int m = arr.size(), total_carni = 0;
    vector<ll> ans(n + 1, INT_MAX); ans[0] = 0;
    for (auto [l, c] : arr) {
        for (int s = n - l; s >= 0; --s) 
            ans[s + l] = min(ans[s + l], ans[s] + (l - c));
        total_carni += c;
    }

    if (ans[total_carni] >= INT_MAX) returnAns(-1)
    returnAns(ans[total_carni])
}



void preComp() {

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

    int t = 1;
    // std::cin >> t;

    for (int i = 1; i <= t; ++i) solve(i);
    return 0;
}