Problem:
Score : 300 points

Problem Statement

Find the maximum possible sum of the digits (in base 10) of a positive integer not greater than N.

Constraints

1\leq N \leq 10^{16}

N is an integer.

Input

Input is given from Standard Input in the following format:

N

Output

Print the maximum possible sum of the digits (in base 10) of a positive integer not greater than N.

Sample Input 1

100

Sample Output 1

18

For example, the sum of the digits in 99 is 18, which turns out to be the maximum value.

Sample Input 2

9995

Sample Output 2

35

For example, the sum of the digits in 9989 is 35, which turns out to be the maximum value.

Sample Input 3

3141592653589793

Sample Output 3

137

Solution in Ruby (2.3.3) (judge verdict: Accepted)

def digits_sum(number)
  if !number.kind_of?(Integer)
    # 整数以外の場合
    puts "[ERROR] digits_sum: argument must be an INTEGER"
    -1
  elsif number < 0
    # 負の整数の場合
    puts "[ERROR] digits_sum: argument must be a POSITIVE integer"
    -1
  else
    # 0もしくは正の整数の場合
    sum = 0
    while number > 0 do
      sum += number % 10
      number /= 10
    end
    sum
  end
end

n = gets.chomp
m = n.split("").map.with_index { |num, i|
  if i == 0
    num.to_i - 1
  else
    9
  end
}.join("").to_i
puts [digits_sum(m), digits_sum(n.to_i)].max