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 C# (.NET Core 3.1.201) (judge verdict: Accepted)

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Text;
using System.Text.RegularExpressions;
using System.Threading.Tasks;

namespace AtCoder
{
	public class QuestionA
	{
		public static void Main(string[] args)
		{
			// 文字列の入力
			string s = Console.ReadLine();

			long result = 0;

			if (s.Length == 1) {
				Console.WriteLine(s);
				return;
			}

			result += (s[0] - '0') - 1;
			result += 9 * (s.Length - 1);

			var simple = digit_sum(long.Parse(s));
			Console.WriteLine(Math.Max(result, simple));
		}

		private static long  digit_sum(long n)
		{
			if (n < 10) return n;
			return digit_sum(n / 10) + n % 10;
		}
	}
}