#include<bits/stdc++.h>
#include<atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i,n)for (int i = 0; i < int(n); ++i)
#define rrep(i,n)for (int i = int(n)-1; i >= 0; --i)
#define all(x) (x).begin(), (x).end()
#define rall(x) (x).rbegin(), (x).rend()
template<class T> void chmax(T& a, const T& b) {a = max(a, b);}
template<class T> void chmin(T& a, const T& b) {a = min(a, b);}
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin >> n;
    int c0 = 0, c1 = 0;
    vector<P> b;
    rep(_, n) {
        int x;
        cin >> x;
        if (x == 0) c0++;
        else if (x == 1) c1++;
        else {
            if (c0 || c1) b.emplace_back(c0, c1);
            c0 = c1 = 0;
        }
    }
    if (c0 || c1) b.emplace_back(c0, c1);
    constexpr int INF = 1001001001;
    vector<int> dp(2 * n + 10, INF);
    const int B = n + 5;
    dp[B] = 0;
    for(auto [c0, c1]: b) {
        vector<int> ndp(2 * n + 10, INF);
        for(int i = -n; i <= n; i++) if (dp[B + i] != INF) {
            chmin(ndp[B + i + c1], dp[B + i] + c1);
            chmin(ndp[B + i - c0], dp[B + i] + c0);
        }
        swap(dp, ndp);
    }
    int ans = dp[B];
    if (ans == INF) ans = -1;
    else ans /= 2;
    cout << ans << '\n';
}