#include <bits/stdc++.h>
// clang-format off
#pragma GCC diagnostic ignored "-Wsign-compare"
#pragma GCC diagnostic ignored "-Wsign-conversion"

#define NDEBUG
using i32 = int32_t;
using i64 = int64_t;
using u32 = uint32_t;
using u64 = uint64_t;
using uint = unsigned int;
using usize = std::size_t;
using ll = long long;
using ull = unsigned long long;
using ld = long double;
template<typename T> constexpr T popcount(const T u) { return u ? static_cast<T>(__builtin_popcountll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T log2p1(const T u) { return u ? static_cast<T>(64 - __builtin_clzll(static_cast<u64>(u))) : static_cast<T>(0); }
template<typename T> constexpr T msbp1(const T u) { return log2p1(u); }
template<typename T> constexpr T lsbp1(const T u) { return __builtin_ffsll(u); }
template<typename T> constexpr T clog(const T u) { return u ? log2p1(u - 1) : static_cast<T>(u); }
template<typename T> constexpr bool ispow2(const T u) { return u and (static_cast<u64>(u) & static_cast<u64>(u - 1)) == 0; }
template<typename T> constexpr T ceil2(const T u) { return static_cast<T>(1) << clog(u); }
template<typename T> constexpr T floor2(const T u) { return u == 0 ? static_cast<T>(0) : static_cast<T>(1) << (log2p1(u) - 1); }
template<typename T> constexpr bool btest(const T mask, const usize ind) { return ((static_cast<u64>(mask) >> ind) & static_cast<u64>(1)); }
template<typename T> constexpr T bcut(const T mask, const usize ind) { return ind == 0 ? static_cast<T>(0) : static_cast<T>((static_cast<u64>(mask) << (64 - ind)) >> (64 - ind)); }
template<typename T> bool chmin(T& a, const T& b) { return (a > b ? a = b, true : false); }
template<typename T> bool chmax(T& a, const T& b) { return (a < b ? a = b, true : false); }
constexpr unsigned int mod                  = 1000000007;
template<typename T> constexpr T inf_v      = std::numeric_limits<T>::max() / 4;
template<typename Real> constexpr Real pi_v = Real{3.141592653589793238462643383279502884};


class xoshiro
{
private:
    using result_type = uint32_t;
    result_type s[4];
    static result_type rotl(const result_type x, const int k) { return (x << k) | (x >> (32 - k)); }

public:
    xoshiro() : xoshiro(std::random_device{}()) {}
    xoshiro(uint64_t seed)
    {
        uint64_t z = 0;
        for (int i = 0; i < 4; i++) { z = (seed += 0x9e3779b97f4a7c15), z = (z ^ (z >> 33)) * 0x62A9D9ED799705F5, z = (z ^ (z >> 28)) * 0xCB24D0A5C88C35B3, s[i] = static_cast<result_type>(z >> 32); }
    }
    result_type operator()()
    {
        const result_type result = rotl(s[1] * 5, 7) * 9, t = s[1] << 9;
        return s[2] ^= s[0], s[3] ^= s[1], s[1] ^= s[2], s[0] ^= s[3], s[2] ^= t, s[3] = rotl(s[3], 11), result;
    }
    void discard(const usize rep)
    {
        for (usize i = 0; i < rep; i++) { (*this)(); }
    }
};
class xoshiro_64
{
private:
    using result_type = uint64_t;
    result_type s[4];
    static result_type rotl(const result_type x, const int k) { return (x << k) | (x >> (64 - k)); }

public:
    xoshiro_64() : xoshiro_64(std::random_device{}()) {}
    xoshiro_64(uint64_t seed)
    {
        uint64_t z = 0;
        for (int i = 0; i < 4; i++) { z = (seed += 0x9e3779b97f4a7c15), z = (z ^ (z >> 30)) * 0xbf58476d1ce4e5b9, z = (z ^ (z >> 27)) * 0x94d049bb133111eb, s[i] = static_cast<result_type>(z ^ (z >> 31)); }
    }
    result_type operator()()
    {
        const result_type result = rotl(s[1] * 5, 7) * 9, t = s[1] << 17;
        return s[2] ^= s[0], s[3] ^= s[1], s[1] ^= s[2], s[0] ^= s[3], s[2] ^= t, s[3] = rotl(s[3], 45), result;
    }
    void discard(const usize rep)
    {
        for (usize i = 0; i < rep; i++) { (*this)(); }
    }
};
template<typename UnsignedInt, typename Rng>
class rng_base
{
public:
    using result_type = UnsignedInt;
    using rng_type    = Rng;
    static_assert(std::is_unsigned<UnsignedInt>::value, "UnsignedInt should be unsigned.");
    rng_base() : rng_base(std::random_device{}()) {}
    rng_base(const u64 seed) : rng(seed) {}
    ~rng_base() = default;
    UnsignedInt operator()() { return rng(); }
    template<typename Int>
    Int uniform_int(const Int max)
    {
        static_assert(std::is_integral<Int>::value, "Integer type is needed.");
        if (ispow2(max + 1)) { return static_cast<Int>(rng() & max); }
        const UnsignedInt mask = static_cast<UnsignedInt>(ceil2(static_cast<u64>(max + 1))) - 1;
        while (true) {
            const Int ans = static_cast<Int>(rng() & mask);
            if (ans <= max) { return ans; }
        }
    }
    template<typename Int> Int uniform_int(const Int min, const Int max) { return min + uniform_int(static_cast<Int>(max - min)); }
    bool uniform_bool() { return uniform_int<bool>(0, 1); }
    template<typename Real>
    Real uniform_real()
    {
        static_assert(std::is_floating_point<Real>::value, "FloatingPoint type is needed.");
        return static_cast<Real>(uniformInt(~static_cast<UnsignedInt>(0)) / static_cast<Real>(~static_cast<UnsignedInt>(0)));
    }
    template<typename Real> Real uniform_real(const Real min, const Real max) { return assert(min <= max), uniform_real<Real>() * (max - min) + min; }
    template<typename Int> std::pair<Int, Int> uniform_int_pair(const Int min, const Int max) { return std::minmax(uniform_int(min, max), uniform_int(min, max)); }
    template<typename Int>
    std::vector<Int> uniform_int_vec(const std::size_t size, const Int min, const Int max)
    {
        std::vector<Int> v(size);
        for (std::size_t i = 0; i < size; i++) { v[i] = uniformInt(min, max); }
        return v;
    }
    template<typename Real>
    std::vector<Real> uniform_real_vec(const std::size_t size, const Real min, const Real max)
    {
        std::vector<Real> v(size);
        for (std::size_t i = 0; i < size; i++) { v[i] = uniformReal(min, max); }
        return v;
    }
    void discard(const usize rep) { rng.discard(rep); }

private:
    Rng rng;
};
using rng_mt        = rng_base<u32, std::mt19937>;
using rng_mt64      = rng_base<u64, std::mt19937_64>;
using rng_xoshiro   = rng_base<u32, xoshiro>;
using rng_xoshiro64 = rng_base<u64, xoshiro_64>;
rng_mt g_rng_mt;
rng_mt64 g_rng_mt64;
rng_xoshiro g_rng_xo;
rng_xoshiro64 g_rng_xo64;
template<typename T>
T read()
{
    T v;
    return std::cin >> v, v;
}
template<typename T>
std::vector<T> read_vec(const std::size_t size)
{
    std::vector<T> v(size);
    for (auto& e : v) { std::cin >> e; }
    return v;
}
#define SHOW(...) static_cast<void>(0)
template<typename T>
std::vector<T> make_v(const std::size_t size, T v) { return std::vector<T>(size, v); }
template<class... Args>
auto make_v(const std::size_t size, Args... args) { return std::vector<decltype(make_v(args...))>(size, make_v(args...)); }

template<typename ValueMonoid>
class segtree
{
public:
    using value_monoid_type = ValueMonoid;
    using value_type        = typename value_monoid_type::value_type;
    segtree(const usize sz, const value_type initial = value_monoid_type::id()) : sz{sz}, half{ceil2(sz)}, val(half << 1, value_monoid_type::id())
    {
        if (initial != value_monoid_type::id()) {
            std::fill(val.begin() + half, val.end(), initial);
            for (usize i = half - 1; i >= 1; i--) { up(i); }
        }
    }
    template<typename InIt>
    segtree(const InIt first, const InIt last) : sz{static_cast<usize>(std::distance(first, last))}, half{ceil2(sz)}, val(half << 1, value_monoid_type::id())
    {
        std::copy(first, last, val.begin() + half);
        for (usize i = half - 1; i >= 1; i--) { up(i); }
    }
    value_type get(const usize a) const { return assert(a < sz), val[a + half]; }
    void set(usize a, const value_type& v)
    {
        assert(a < sz);
        val[a += half] = v;
        while (a >>= 1) { up(a); }
    }
    value_type fold(usize l, usize r) const
    {
        assert(l < r), assert(r <= sz);
        value_type accl = value_monoid_type::id(), accr = value_monoid_type::id();
        for (l += half, r += half; l < r; l >>= 1, r >>= 1) {
            if (l & 1) { accl = value_monoid_type::merge(accl, val[l++]); }
            if (r & 1) { accr = value_monoid_type::merge(val[--r], accr); }
        }
        return value_monoid_type::merge(accl, accr);
    }
    usize size() const { return sz; }
    friend std::ostream& operator<<(std::ostream& os, const segtree& seg)
    {
        os << "[";
        for (usize i = seg.half; i < seg.half + seg.size; i++) { os << seg.val[i] << (i + 1 == seg.half + seg.size ? "" : ","); }
        return (os << "]\n");
    }

private:
    void up(const usize i) { val[i] = value_monoid_type::merge(val[i << 1], val[i << 1 | 1]); }
    const usize sz, half;
    std::vector<value_type> val;
};

template<typename Element>
struct min
{
    using element_type = Element;
    using value_type   = element_type;
    min()              = delete;
    static value_type merge(const value_type& a, const value_type& b) { return std::min(a, b); }
    static constexpr value_type id() { return inf_v<value_type>; }
};
// clang-format on
int main()
{
    const auto n = read<usize>();
    const auto s = read<std::string>();
    std::vector<usize> nzero(n);
    for (usize j = 0, nz = n; j < n; j++) {
        const usize i = n - j - 1;
        if (s[i] == '0') { nz = i; }
        nzero[i] = nz;
    }
    SHOW(nzero);
    std::vector<usize> dp(n + 1, 0);
    for (usize i = 0; i < n; i++) {
        SHOW(i);
        chmax(dp[i + 1], dp[i]);
        const usize nz = nzero[i];
        if (s[i] == '1' and nz < n - 1 and s[nz + 1] == '1') {
            const usize ol = nzero[i] - i;
            chmax(dp[nz + 2], dp[i] + ol);
            if (ol == 1) {
                for (int o = nz + 1; o < n and s[o] == '1'; o++) { chmax(dp[o + 1], dp[i] + o - nz); }
            }
        }
    }
    std::cout << dp[n] << std::endl;
    return 0;
}


Measured memory_bytes: 8708