#include <bits/stdc++.h>
using namespace std;
using ll = long long;
template<typename T> using vc = vector<T>;
template<typename T> using vv = vc<vc<T>>;
using vl = vc<ll>;
using vs = vc<string>;
#define rep(i,n) for(ll i=0;i<(n);i++)
#define pb push_back
#define pob pop_back
#define YES cout<<"Yes"<<endl
#define NO cout<<"No"<<endl
#define YN {cout<<"Yes"<<endl;}else{cout<<"No"<<endl;}
#define M1 cout<<"-1"<<endl
#define INF (ll)4e18
vl dx = {1,-1,0,0};vl dy = {0,0,1,-1};
int main(){
    string s;
    cin >> s;
    ll count=0;
    rep(i,s.size()){
        if(s[i]=='1' && s[i+1]=='1' && s[i+2]=='0'){
            count++;
            swap(s[i],s[i+2]);
        //cout << s << endl;
            if(i>=5)i-=5;
        }
    }
    cout << count << endl;
}