#include<bits/stdc++.h>
using namespace std;
#define fastIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);
typedef long long ll;
int main()
{
  fastIO
  int ans=0;
  string s;cin>>s;
  for(int i=0;i<s.size();i++)
  {
    if(s[i]=='1')ans++;
    else if(s[i]=='0'&&(s[i-1]=='1'&&s[i+1]=='1'))ans++;
  }
  cout<<ans<<"\n";
}