MultiplyBigNumber

using System;
using System.Text;

class MultiplyBigNumber
{
    static void Main()
    {
        string number = Console.ReadLine();
        int multiplier = int.Parse(Console.ReadLine());

        var sb = new StringBuilder();

        int reminder = 0;

        for (int i = number.Length - 1; i >= 0; i--)
        {
            if (multiplier == 0)
            {
                sb.Append(0);
                break;
            }
            else if (multiplier == 1)
            {
                sb.Append(number);
                break;
            }

            int product = reminder +
                int.Parse(number[i].ToString()) * multiplier;

            int digit = product % 10;
            sb.Insert(0, digit);

            reminder = product / 10;

            if (i == 0 && reminder != 0)
            {
                sb.Insert(0, reminder);
            }
        }

        string result = sb.ToString().TrimStart('0');
        Console.WriteLine(result);
    }
}