#include<bits/stdc++.h>//万能库
using
namespace
std;
int
main()
{
n,a,b=0,c,i;
cin>>n;
for
(i=1;i<=n;i++)
i++;
a=i;
while
(a!=0)
c=a%10;
if
(c==1)
b++;
a=a/10;
}
cout<<b;
return
0;