package main

import (
	"fmt"
)

func main() {
	var N int
	fmt.Scan(&N)

	m := 10
	count := 0
	for n := 1; ; n++ {
		if m <= n {
			m *= 10
		}

		nnn := n*m*m + n*m + n
		if nnn > N {
			break
		}
		count++
	}

	fmt.Println(count)
}