#include <bits/stdc++.h>
using namespace std;
#define maxn 500005
int n,last[maxn],dp[maxn];
char s[maxn];
int main()
{	
	scanf("%d%s",&n,s+1);
	int lp=0;
	for (int i=1;i<=n;i++)
	{
		last[i]=lp;
		if (s[i]=='0') lp=i;
	}
	for (int i=1;i<=n;i++)
	{
		dp[i]=dp[i-1];
		if (s[i]=='0' || !last[i] || s[last[i]-1]=='0') continue;
		int pos=last[i];
		int lpos=last[pos];
		if (pos>=2) dp[i]=max(dp[i],dp[pos-2]+i-pos);//10111111...
		if (pos==i-1)//0111111...01
		{
			if (lpos>=0) dp[i]=max(dp[i],dp[lpos]+i-lpos-2);//dp[lpos]+number of 1 on the left
		if (s[lpos-1]=='1')	if (lpos+1<pos) dp[i]=max(dp[i],dp[lpos+1]+i-lpos-3);
		}
	}
	printf("%d\n",dp[n]);
	return 0;
}
