#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;
//typedef pair<ll,ll> PLL;
ll xi[lens];
const ll M = 1e9+7;
ll yi[lens];
//ll bo[lens];
ll l,r,y;
ll n,m,r1,r2,i,j,sum2,cn,d,x0,y0,k;
string na,te,ss,s;

void sol(){
    cin>>s;
    stack<char> st;
    string gd="";
    string ans="";
    for (ll i = 0; i < s.size(); ++i){
        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 (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){
                continue;
            }
            else{
                if (ans[i+1]=='1'&&ans[i-1]=='1'){
                    rq++;
                }
            }
        }
    }
    cout<<rq<<"\n";
    /*
    for (ll i = 0; i < s.size(); ++i) {
        if (st.empty()){
            if (s[i]=='0'){
                continue;
            }
            else{
                st.push(s[i]);
            }
        }
        else{
            if (s[i]=='0'){
                
            }
            else{
                st.push(s[i]);
            }
        }
    }
    */

    
    /*
    for (i=0;i<n;i++){

    }
    */
    //cout<<"Yes\n";
    //cout<<"No\n";   
}
int main(){  
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
    ll t=1;
    //cin>>t;
    while (t--){
        sol();
    }
    return 0;  
}

answer.code:33:31: error: ‘ll y0’ redeclared as different kind of entity
   33 | ll n,m,r1,r2,i,j,sum2,cn,d,x0,y0,k;
      |                               ^~
In file included from /usr/include/features.h:461,
                 from /usr/include/x86_64-linux-gnu/c++/13/bits/os_defines.h:39,
                 from /usr/include/x86_64-linux-gnu/c++/13/bits/c++config.h:679,
                 from /usr/include/c++/13/bits/requires_hosted.h:31,
                 from /usr/include/c++/13/iostream:38,
                 from answer.code:1:
/usr/include/x86_64-linux-gnu/bits/mathcalls.h:220:1: note: previous declaration ‘double y0(double)’
  220 | __MATHCALL (y0,, (_Mdouble_));
      | ^~~~~~~~~~