/* #region  header */
#pragma GCC optimize("O3") //コンパイラ最適化用

#ifdef LOCAL
#define _GLIBCXX_DEBUG //配列に[]でアクセス時のエラー表示
#endif
#define _USE_MATH_DEFINES
#include <algorithm> //sort,二分探索,など
#include <bitset>    //固定長bit集合
// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
#include <cassert> //assert(p)で，not pのときにエラー
#include <cctype>
#include <chrono> //実行時間計測
#include <climits>
#include <cmath>   //pow,logなど
#include <complex> //複素数
#include <cstdio>
#include <cstring>
#include <deque>
#include <functional> //sortのgreater
#include <iomanip>    //setprecision(浮動小数点の出力の誤差)
#include <ios>        // std::left, std::right
#include <iostream>   //入出力
#include <iterator>   //集合演算(積集合,和集合,差集合など)
#include <map>
#include <numeric> //iota(整数列の生成),gcdとlcm,accumulate
#include <queue>
#include <random>
#include <set>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility> //pair
#include <vector>
using namespace std;
typedef long long LL;
typedef long double LD;

#define ALL(x) x.begin(), x.end()
const long long INF = numeric_limits<long long>::max() / 4;
const int MOD = 1e9 + 7;
// const int MOD=998244353;
//略記
#define FF first
#define SS second
#define int long long
#define stoi stoll
#define LD long double
#define PII pair<int, int>
#define PB push_back
#define EB emplace_back
#define MP make_pair
#define SZ(x) (int)((x).size())
#define VB vector<bool>
#define VVB vector<vector<bool>>
#define VI vector<int>
#define VVI vector<vector<int>>

#define REP(i, n) for (int i = 0; i < (int)(n); i++)
#define REPD(i, n) for (int i = (int)(n) - (int)1; i >= 0; i--)
#define FOR(i, a, b) for (int i = a; i < (int)(b); i++)
#define FORD(i, a, b) for (int i = (int)(b) - (int)1; i >= (int)a; i--)

const int dx[4] = {0, 1, 0, -1}, dy[4] = {-1, 0, 1, 0};
const int Dx[8] = {0, 1, 1, 1, 0, -1, -1, -1},
          Dy[8] = {-1, -1, 0, 1, 1, 1, 0, -1};

int in() {
    int x;
    cin >> x;
    return x;
}
// https://qiita.com/Lily0727K/items/06cb1d6da8a436369eed#c%E3%81%A7%E3%81%AE%E5%AE%9F%E8%A3%85
void print() { cout << "\n"; }
template <class Head, class... Tail> void print(Head &&head, Tail &&...tail) {
    cout << head;
    if (sizeof...(tail) != 0)
        cout << " ";
    print(forward<Tail>(tail)...);
}
template <class T> void print(vector<T> &vec) {
    for (auto &a : vec) {
        cout << a;
        if (&a != &vec.back())
            cout << " ";
    }
    cout << "\n";
}
template <class T> void print(set<T> &set) {
    for (auto &a : set) {
        cout << a << " ";
    }
    cout << "\n";
}
template <class T> void print(vector<vector<T>> &df) {
    for (auto &vec : df) {
        print(vec);
    }
}

// debug macro
// https://atcoder.jp/contests/abc202/submissions/22815715
namespace debugger {
template <class T> void view(const std::vector<T> &a) {
    std::cerr << "{ ";
    for (const auto &v : a) {
        std::cerr << v << ", ";
    }
    std::cerr << "\b\b }";
}
template <class T> void view(const std::vector<std::vector<T>> &a) {
    std::cerr << "{\n";
    for (const auto &v : a) {
        std::cerr << "\t";
        view(v);
        std::cerr << "\n";
    }
    std::cerr << "}";
}
template <class T, class U> void view(const std::vector<std::pair<T, U>> &a) {
    std::cerr << "{\n";
    for (const auto &p : a)
        std::cerr << "\t(" << p.first << ", " << p.second << ")\n";
    std::cerr << "}";
}
template <class T, class U> void view(const std::map<T, U> &m) {
    std::cerr << "{\n";
    for (const auto &p : m)
        std::cerr << "\t[" << p.first << "] : " << p.second << "\n";
    std::cerr << "}";
}
template <class T, class U> void view(const std::pair<T, U> &p) {
    std::cerr << "(" << p.first << ", " << p.second << ")";
}
template <class T> void view(const std::set<T> &s) {
    std::cerr << "{ ";
    for (auto &v : s) {
        view(v);
        std::cerr << ", ";
    }
    std::cerr << "\b\b }";
}

template <class T> void view(const T &e) { std::cerr << e; }
} // namespace debugger
#ifdef LOCAL
void debug_out() {}
template <typename Head, typename... Tail> void debug_out(Head H, Tail... T) {
    debugger::view(H);
    std::cerr << ", ";
    debug_out(T...);
}
#define debug(...)                                                             \
    do {                                                                       \
        std::cerr << __LINE__ << " [" << #__VA_ARGS__ << "] : [";              \
        debug_out(__VA_ARGS__);                                                \
        std::cerr << "\b\b]\n";                                                \
    } while (false)
#else
#define debug(...) (void(0))
#endif

long long power(long long x, long long n) {
    // O(logn)
    // https://algo-logic.info/calc-pow/#toc_id_1_2
    long long ret = 1;
    while (n > 0) {
        if (n & 1)
            ret *= x; // n の最下位bitが 1 ならば x^(2^i) をかける
        x *= x;
        n >>= 1; // n を1bit 左にずらす
    }
    return ret;
}
// @brief nCr. O(r log n)。あるいは前処理 O(n), 本処理 O(1)で求められる modint
// の bc を検討。
int comb(int n, int r) {
    // https://www.geeksforgeeks.org/program-to-calculate-the-value-of-ncr-efficiently/
    if (n < r)
        return 0;

    // p holds the value of n*(n-1)*(n-2)...,
    // k holds the value of r*(r-1)...
    long long p = 1, k = 1;

    // C(n, r) == C(n, n-r),
    // choosing the smaller value
    if (n - r < r)
        r = n - r;

    if (r != 0) {
        while (r) {
            p *= n;
            k *= r;

            // gcd of p, k
            long long m = __gcd(p, k);

            // dividing by gcd, to simplify
            // product division by their gcd
            // saves from the overflow
            p /= m;
            k /= m;

            n--;
            r--;
        }

        // k should be simplified to 1
        // as C(n, r) is a natural number
        // (denominator should be 1 ) .
    }

    else
        p = 1;

    // if our approach is correct p = ans and k =1
    return p;
}

// MOD
void add(long long &a, long long b) {
    a += b;
    if (a >= MOD)
        a -= MOD;
}
template <class T> inline bool chmin(T &a, T b) {
    if (a > b) {
        a = b;
        return true;
    }
    return false;
}
template <class T> inline bool chmax(T &a, T b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

// 負数も含む丸め
long long ceildiv(long long a, long long b) {
    if (b < 0)
        a = -a, b = -b;
    if (a >= 0)
        return (a + b - 1) / b;
    else
        return a / b;
}
long long floordiv(long long a, long long b) {
    if (b < 0)
        a = -a, b = -b;
    if (a >= 0)
        return a / b;
    else
        return (a - b + 1) / b;
}
long long floorsqrt(long long x) {
    assert(x >= 0);
    long long ok = 0;
    long long ng = 1;
    while (ng * ng <= x)
        ng <<= 1;
    while (ng - ok > 1) {
        long long mid = (ng + ok) >> 1;
        if (mid * mid <= x)
            ok = mid;
        else
            ng = mid;
    }
    return ok;
}

// @brief a^n mod mod
long long modpower(long long a, long long n, long long mod) {
    long long res = 1;
    while (n > 0) {
        if (n & 1)
            res = res * a % mod;
        a = a * a % mod;
        n >>= 1;
    }
    return res;
}

// @brief s が c を含むか
template <class T> bool contain(const std::string &s, const T &c) {
    return s.find(c) != std::string::npos;
}

__attribute__((constructor)) void faster_io() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    cerr.tie(nullptr);
}
/* #endregion */

int n;

bool check(vector<PII> &B) {
    REP(ni, SZ(B)) {
        if (B[ni].FF != 0 and B[ni].SS != 0)
            return false;
    }
    return true;
}

signed main() {
    cin >> n;
    VI A(n);
    REP(ni, n) { cin >> A[ni]; }
    int now0 = 0, now1 = 0;
    vector<PII> B;
    REP(ni, n) {
        if (A[ni] == 0)
            now0++;
        else if (A[ni] == 1)
            now1++;
        else {
            if (!(now0 == 0 and now1 == 0)) {
                B.PB(MP(now0, now1));
                now0 = 0, now1 = 0;
            }
        }
    }
    if (!(now0 == 0 and now1 == 0)) {
        B.PB(MP(now0, now1));
    }

    sort(ALL(B));
    debug(B);
    if (check(B)) {
        print(0);
        exit(0);
    }
    int m = SZ(B);
    int ans = 0;
    int r = m - 1;
    bool ng = false;
    REP(mi, m) {
        debug(mi);
        debug(B);
        if (check(B)) {
            print(ans);
            exit(0);
        }
        if (mi >= r) {
            ng = true;
            break;
        }

        int &lnum = B[mi].FF;
        while (lnum > 0) {
            int minus = min(lnum, B[r].SS);
            ans += minus;
            lnum -= minus, B[r].SS -= minus;
            if (B[r].SS == 0) {
                r--;
                if (lnum > 0 and mi >= r) {
                    ng = true;
                    break;
                }
            }
        }
        if (ng)
            break;
    }
    if (ng) {
        print(-1);
    } else {
        print(ans);
    }

    return 0;
}