#include <iostream>
#include <string>
#include <sstream>
#include <vector>
#include <algorithm>
#include <bits/stdc++.h>
#include <numeric>
#include <stack>
#include <map>
#include <iterator>
#include <iomanip>
#include <cstdlib>
#include <ctime>
#include <cctype>
#include <cmath>
#include <bitset>
using namespace std;  
typedef long long ll;
#define pb push_back
typedef double db;
#define vel vector<long long>
#define ves vector<string>
typedef unsigned long long ull;
const double pi= std::acos(-1.0);
const ll lens=1e4+5;
const ll MAXN5=1e5+5;

void sol(){
    string s;
	ll sum=0;
    cin>>s;
    string gd="";
    string ans="";
    for (ll i = 0; i < s.size(); ++i){
		sum+=s[i]-'0';
        if (gd.size()==0){
            gd+=s[i];
        }
        else{
            if (s[i]==gd[gd.size()-1]){
                gd+=s[i];
            }
            else{
                if (ans.size()==0){
                    if (gd[0]=='1'){
                        ans+='1';
                    }
                }
                else{
                    ans+=gd[0];
                }
                gd="";
                gd+=s[i];
            }
        }
    }
	if (sum==0||sum==s.size()){
		cout<<0;
		return;
	}
    if (gd.size()!=0){
        if (gd[0]=='1'){
            if (ans[ans.size()-1]=='0'){
                ans+='1';
            }
        }
    }
    ll rq=0;
    for (ll i = 0; i < ans.size(); ++i){
        if (ans[i]=='0'){
            if (i==ans.size()-1||i==0){
                continue;
            }
            else{
                if (ans[i+1]=='1'&&ans[i-1]=='1'){
                    rq++;
                }
            }
        }
    }
    cout<<rq<<"\n"; 
}
int main(){  
    ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
   
    ll t=1;
    //cin>>t;
    while (t--){
        sol();
    }
    return 0;  
}