#include <bits/stdc++.h>

using namespace std;

typedef pair<int,int> ii;

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

  int n;
  cin >> n;
  string s;
  cin >> s;
  vector<int> from(n), to(n);
  to[0] = (s[0] == '1');
  from[n - 1] = (s[n - 1] == '1');
  for (int i = 1; i < n; i++) to[i] = (s[i] == '1') ? to[i - 1] + 1 : 0;
  for (int i = n - 2; i >= 0; i--) from[i] = (s[i] == '1') ? from[i + 1] + 1 : 0;
  
  vector<vector<int>> blocks(n);
  for (int i = 1; i < n - 1; i++) {
    if (s[i] == '0' && s[i - 1] == '1' && s[i + 1] == '1') {
      for (int j = 1; j <= to[i - 1]; j++) {
        blocks[i + 1].push_back(j);
      }
      for (int j = 1; j <= from[i + 1]; j++) {
        blocks[i + j].push_back(j);
      }
    } 
  }

  vector<int> dp(n);
  for (int i = 1; i < n; i++) {
    dp[i] = dp[i - 1];
    for (int b : blocks[i]) {
      if (i - b - 2 >= 0) dp[i] = max(dp[i], dp[i - b - 2] + b);
      else dp[i] = max(dp[i], b);
    }
  }
  cout << dp[n - 1] << endl;

  return 0;
}
