#include<bits/stdc++.h>
using namespace std;
using ll = long long;
#define all(a) begin(a) end(a)
#define rep(i,a,b) for(ll i = a; i < (b); i++)

int main(){
    string s;
    cin>> s;
    ll ans = 0;
    ll cnt = 0;
    rep(i,0,s.size()){
        if(s[i] == '0'){
            ans += cnt/2;
            if(cnt % 2) cnt--;
        }else{
            cnt++;
        }
    }
    cout << ans << endl;
}