#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for (int i = 0; i < (int)(n); i++)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<long long> VL;
typedef vector<VL> VVL;
typedef long long LL;
#define all(a) (a).begin(), (a).end()
#define Yes(n) cout << ((n) ? "Yes" : "No"  ) << endl
#define ALL(a)  (a).begin(),(a).end()
#define pb push_back

int main() {
  string s;cin>>s;
  int n=s.size();
  LL ans=0;
  int a=0;
  rep(i,n){
    if(s[i]=='1'){a++;}
    else{
      ans+=a/2;
      a=a/2*2;
    }
  }
  cout<<ans<<endl;
}