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

int main () {
  string s;
  cin >> s;
  
  int ans = 0;
  int one_count = 0;
  for(int i = 0; i < s.size(); i++){
    if(s[i] == '1'){
      if(one_count > 0){
        one_count++;
      } else if(s[i + 1] == '1') {
        one_count++;
      }
    } else {
      ans += one_count / 2;
    }
  }
  
  cout << ans << endl;
}