#include <bits/stdc++.h>
using namespace std;
#ifdef local
#include <debug.hpp>
#else
#define debug(...)
#endif
#define rep(i, n) for (int i = 0; i < n; i++)
template <class T> istream& operator>>(istream& I, vector<T>& V) {for (T& X : V) I >> X; return I;}
template <class T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return true;} return false;}
template <class T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return true;} return false;}
vector<int> di = {-1, 1, 0, 0}, dj = {0, 0, -1, 1}; int inf = 2e9; long INF = 1e18;

int main() {
    string s;
    cin >> s;
    int n = s.size();

    int x = 0;
    long ans = 0;
    for (int i = n - 1; i > 0; i--) {
        if (s[i] == '1') {
            if (s[i - 1] == '1') {
                ans += x;
                i--;
            } else {
                x = 0;
            }
        } else {
            x++;
        }
    }
    cout << ans << endl;
    return 0;
}