题解 | #自守数#

using System;

namespace HJ99
{
    internal class Program
    {
        static void Main(string[] args)
        {
            int n = int.Parse(Console.ReadLine());

            int count = 0;
            for (int i = 0; i <= n; i++)
            {
                int m = i * i - i;
                int power = i < 10 ? 1 : ((int)Math.Log10(i) + 1);
                double compare = Math.Pow(10, power);
                if (m % compare == 0)
                {
                    count++;
                }
            }
            Console.WriteLine(count);
        }
    }
}