#include <bits/stdc++.h>
#define pb push_back
using namespace std;
typedef long ll;
const int MAXN = 600005;

int dp[MAXN];
int main()
{
//	freopen("input.in", "r", stdin);
    int n;
    cin >> n;
    string s;
    cin >> s;

    int zero2 = 0, zero1 = 1;
    int i;
    s = "00" + s;
    dp[0] = 0;
    dp[1] = 0;
    for (int i = 2; i < s.length(); i++)
    {
        dp[i] = dp[i-1];

        if (s[i] == '0')
        {
            zero2 = zero1;
            zero1 = i;
        }
        else
        {
            if (s[i-1] == '0')
            {
                dp[i] = max(dp[i], dp[zero2] + i - zero2 - 2);
                dp[i] = max(dp[i], dp[zero2+1] + i - (zero2+1) - 2);
            }
            else if (s[zero1-1] == '1')
                dp[i] = max(dp[i], dp[zero1-2] + i - (zero1-2) - 2);
        }
    }
    cout << dp[s.length()-1];
	return 0;
}
