#include <bits/stdc++.h>
#include <stdio.h>
using namespace std;

#define INF 1.1e9
#define LINF 1.1e18
#define FOR(i,a,b) for (int i = (a); i < (b); ++i)
#define REP(i,n) FOR(i,0,n)
#define ALL(v) (v).begin(), (v).end()

typedef long long ll;
typedef pair<int, int> P;

int score[500100];
int n,last;
char s[500100];

int main(){
	cin.tie(0);
  ios::sync_with_stdio(false);

	cin>>n>>s;
	FOR(i,2,n) {
		score[i]=score[i-1];
		if(s[i-2]=='1'&&s[i-1]=='0'&&s[i]=='1') {
			last=i;
			for(int j=i-2;(s[j]!='0'&&j>=0);--j) score[i]=max(score[i],score[j-1]+(i-j-1));
		}
		if(s[i]=='0') last=0;
		if(last!=0) score[i]=max(score[i],score[last-3]+(i-last+1));
	}
	cout<<score[n-1]<<endl;

	return 0;
}
