using System;
using System.IO;
using System.Linq;
using System.Text;
using System.Collections.Generic;
using System.Diagnostics;
using Enu = System.Linq.Enumerable;

public class Program
{
    public void Solve()
    {
        int N = Reader.Int();
        var S = Reader.String();
        var A = S.Select(c => c - '0').ToArray();
        var Left = new List<long>();
        var Right = new List<long>();

        for (int i = 0; i + 2 < N; i++)
            if (A[i] == 1 && A[i + 1] == 0 && A[i + 2] == 1)
            {
                int L = i - 1, R = i + 3;
                while (L >= 0 && A[L] == 1) L--;
                while (R < N && A[R] == 1) R++;
                Right.Add(((long)(L + 1) << 32) + (i + 3));
                Left.Add(((long)i << 32) + R);
            }
        Left.Sort();
        Right.Sort();
        var dp = new int[N + 1];
        int nextRight = 0, Li = 0, Ri = 0;
        for (int i = 0; i < N; i++)
        {
            CheckMax(ref dp[i + 1], dp[i]);
            if (Ri < Right.Count && (int)(Right[Ri] >> 32) == i)
                nextRight = (int)Right[Ri++];
            if (nextRight >= i + 3)
                CheckMax(ref dp[nextRight], dp[i] + nextRight - i - 2);
            if (Li < Left.Count && (int)(Left[Li] >> 32) == i)
            {
                int R = (int)Left[Li++];
                for (int next = i + 3; next <= R; next++)
                    CheckMax(ref dp[next], dp[i] + (next - i - 2));
            }
        }

        Console.WriteLine(dp[N]);
    }

    bool CheckMax(ref int a, int b) { if (b > a) { a = b; return true; } return false; }
}


class Entry { static void Main() { new Program().Solve(); } }
class Reader
{
    static TextReader reader = Console.In;
    static readonly char[] separator = { ' ' };
    static readonly StringSplitOptions op = StringSplitOptions.RemoveEmptyEntries;
    static string[] A = new string[0];
    static int i;
    static void Init() { Dispose(); A = new string[0]; }
    public static void Set(TextReader r) { Init(); reader = r; }
    public static void Set(string file) { Init(); reader = new StreamReader(file); }
    public static bool HasNext() { return CheckNext(); }
    public static string String() { return Next(); }
    public static int Int() { return int.Parse(Next()); }
    public static long Long() { return long.Parse(Next()); }
    public static double Double() { return double.Parse(Next()); }
    public static int[] IntLine() { return Array.ConvertAll(Split(Line()), int.Parse); }
    public static int[] IntArray(int N) { return Range(N, Int); }
    public static int[][] IntTable(int H) { return Range(H, IntLine); }
    public static string[] StringArray(int N) { return Range(N, Next); }
    public static string[][] StringTable(int N) { return Range(N, () => Split(Line())); }
    public static string Line() { return reader.ReadLine().Trim(); }
    public static T[] Range<T>(int N, Func<T> f) { var r = new T[N]; for (int i = 0; i < N; r[i++] = f()) ; return r; }
    public static void Dispose() { reader.Dispose(); }
    static string[] Split(string s) { return s.Split(separator, op); }
    static string Next() { CheckNext(); return A[i++]; }
    static bool CheckNext()
    {
        if (i < A.Length) return true;
        string line = reader.ReadLine();
        if (line == null) return false;
        if (line == "") return CheckNext();
        A = Split(line);
        i = 0;
        return true;
    }
}

Measured memory_bytes: 24416