// clang-format off
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

#define mp make_pair
#define fst first
#define snd second
#define forn(i,n) for (int i = 0; i < int(n); i++)
#define forn1(i,n) for (int i = 1; i <= int(n); i++)
#define popcnt __builtin_popcountll
#define ffs __builtin_ffsll
#define ctz __builtin_ctzll
#define clz __builtin_clz
#define clzll __builtin_clzll
#define all(a) (a).begin(), (a).end()

using namespace std;
using namespace __gnu_pbds;

using uint = unsigned int;
using ll = long long;
using ull = unsigned long long;
using pii = pair<int,int>;
using pli = pair<ll,int>;
using pil = pair<int,ll>;
using pll = pair<ll,ll>;
template <typename T> using vec = vector<T>;
using vi = vec<int>;
using vl = vec<ll>;
template <typename T> using que = queue<T>;
template <typename T> using deq = deque<T>;
template <typename T> using ordered_set = tree<T, null_type, less<T>, rb_tree_tag, tree_order_statistics_node_update>;
template <typename K, typename V> using ordered_map = tree<K, V, less<K>, rb_tree_tag, tree_order_statistics_node_update>;

template <typename T> T id(T b) {return b;};
template <typename T> void chmax(T &x, T y) {if (x < y) x = y;}
template <typename T> void chmin(T &x, T y) {if (x > y) x = y;}
template <typename S, typename K> bool contains(S &s, K k) { return s.find(k) != s.end(); }
template <typename T> bool getf(T flag, size_t i) { return (flag>>i) & 1; }
template <typename T> T setf(T flag, size_t i) { return flag | (T(1)<<i); }
template <typename T> T unsetf(T flag, size_t i) { return flag & ~(T(1)<<i); }
void fastio() { ios_base::sync_with_stdio(false); cin.tie(nullptr); }
constexpr ll TEN(int n) { if (n == 0) return 1LL; else return 10LL*TEN(n-1); }
// clang-format on

int main() {
    fastio();

    int n;
    cin >> n;
    vi cnt0, cnt1;
    int c0 = 0, c1 = 0;
    forn(i, n) {
        int x;
        cin >> x;
        if (x == 0)
            c0++;
        else if (x == 1)
            c1++;
        else {
            if (c0 + c1 > 0) {
                cnt0.push_back(c0);
                cnt1.push_back(c1);
            }
            c0 = 0;
            c1 = 0;
        }
    }
    if (c0 + c1 > 0) {
        cnt0.push_back(c0);
        cnt1.push_back(c1);
    }

    const int INF = 11111;
    int base = accumulate(all(cnt1), 0);

    int m = cnt0.size();

    vec<vi> dp(m + 1, vi(n + 1, INF)); // dp[i][j]: i番目までの収支がjのときの最小操作回数
    dp[0][base] = 0;

    forn1(i, m) {
        forn(j, n + 1) {
            if (dp[i - 1][j] == INF) continue;

            // i番目のグループをすべて肉食動物にする
            int k1 = j - cnt1[i - 1];
            chmin(dp[i][k1], dp[i - 1][j] + cnt1[i - 1]);

            // i番目のグループをすべて草食動物にする
            int k2 = j + cnt0[i - 1];
            chmin(dp[i][k2], dp[i - 1][j] + cnt0[i - 1]);
        }
    }

    int ans = 0;
    if (dp[m][base] == INF) {
        ans = -1;
    } else {
        ans = dp[m][base] / 2;
    }

    cout << ans << endl;

    return 0;
}