
#include <bits/stdc++.h>
using namespace std;
const int MN = 1e6;
int dp[MN];
	string s;
	int n;
int main() {

	cin>>n;
	cin>>s;
	int ans = 0;
	for(int i = 0; i < n; ++i) {
		dp[i+1] = max(dp[i], dp[i+1]);
		if(i + 2 < n && s[i] == '1' && s[i+1] == '0' && s[i+2] == '1') {
			int pos = i;
			while(pos > 0) {
				if(s[pos] != '1') {
					break;
				}
				--pos;
			}
			if(s[pos] != '1') ++pos;
			dp[i+2+1] = max(dp[i+2+1], dp[pos+1-1] + i - pos + 1);
			if(i - pos +1 > 1) {
				dp[i+2+1] = max(dp[i+2+1], dp[pos+1-1+1] + i - pos);
			}
			pos = i+2;
			while(pos+1 < n) {
				if(s[pos] != '1') break;
				++pos;
			}
			if(s[pos] != '1') --pos;
			dp[pos+1] = max(dp[pos+1], dp[i] + pos-(i+2)+1);
			if(pos - (i+2) + 1 > 1) {
				dp[pos] = max(dp[pos], dp[i] + pos-(i+2));

			 }
		}
	}
	cout<<dp[n]<<'\n';
}
