#include<bits/stdc++.h>
#define ll long long
#define fi first
#define se second
using namespace std;
struct node 
{
	ll l,r;
}num[105];
mt19937_64 rng(chrono::steady_clock::now().time_since_epoch().count());
int main()
{
	ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
	string s;
	cin>>s;
	ll l=0;
	ll ans=0;
	while(l<=s.size()-1)
	{
		ll r=l;
		while(r<=s.size()-1&&s[r]=='1')
		{
			r++;
		}
		if(r!=l)
		{
			num[++ans].l=l;
			num[ans].r=r-1;
			l=r;
		}
		l++;
	}
	ll cur=0;
	for(ll i=ans;i>=2;i--)
	{
		ll u1=num[i].r-num[i].l+1;
		ll u2=num[i].l-num[i-1].r-1;
		cur+=min(u1,u2);
	}
	cout<<cur<<"\n";
}