#include <cstdio>
#include <cstring>
#include <iostream>
#include <string>
#include <cmath>
#include <bitset>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <deque>
#include <algorithm>
using namespace std;
typedef long long int ll;
typedef pair<int, int> P;

int main()
{
	int n;
	cin>>n;
	string s;
	cin>>s;
	int dp[500001];
	dp[0]=0, dp[1]=0;
	int i0=-1;
	if(s[1]=='0') i0=1;
	for(int i=2; i<n; i++){
		dp[i]=dp[i-1];
		if(s[i]=='1'){
			if(i0==-1 || s[i0-1]=='0') continue;
			if(i0==i-1){
				for(int j=i-2; j>=0; j--){
					if(s[j]=='0') break;
					if(j>0) dp[i]=max(dp[j-1]+i-1-j, dp[i]);
                  else dp[i]=max(i-1, dp[i]);
				}
			}else{
				if(i0==1) dp[i]=max(i-1, dp[i]);
				else dp[i]=max(dp[i0-2]+i-i0, dp[i]);
			}
		}else{
			i0=i;
		}
	}
	cout<<dp[n-1]<<endl;
	return 0;
}