//制約チェック
#include <iostream>
#include <algorithm>
#include <atcoder/all>
#include <cassert>
using namespace std;
using namespace atcoder;
using ll = long long;
string S;

int main(){
  cin >> S;
  int sizeS = (int)S.size();
  assert(1 <= sizeS && sizeS <= 2000000);
  ll ans = 0;
  ll one = 0;
  for(int i = 0; i < sizeS; i++){
    assert(S[i] == '0' || S[i] == '1');
    if(S[i] == '0'){
      ans += one / 2;
      one -= one % 2;
    }else{
      one++;
    }
  }
  cout << ans << endl;
  return 0;
}