import java.util.*;
class Main{
public static void main
(String[] args
){
Scanner sc
= new Scanner
(System.
in);
int N = sc.nextInt();
int power = 0;
while(N>=10){
power++;
N /= 10;
}
int ans = 0;
for(int i=1; i<=power; i++){
if(i%2 == 1){
ans
+= 9*Math.
pow(10,i
-1);
}
}
if(power%2 == 1){
}
else{
System.
out.
println(ans
+(N
+1-1*Math.
pow(N,power
)));
}
}
}
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