#include <bits/stdc++.h>
#include <atcoder/all>
using mint = atcoder::static_modint<998244353>;
// using mint = atcoder::static_modint<1000000007>;
using namespace std;
using namespace atcoder;
using ld = long double;
using ll = long long;
#define mp(a,b) make_pair(a,b)
#define rep(i,s,n) for(int i=s; i<n; i++)
const vector<int> dx{1,0,-1,0},dy{0,1,0,-1};

int main(){
    string s;cin >> s;
    reverse(s.begin(),s.end());
    int n=s.size();
    int ans=0;
    rep(i,0,n-2){
        if(s.substr(i,3)=="011"){
            ans++;
            swap(s[i],s[i+2]);
        }
    }
    cout << ans;
}