15. HexadecimalToDecimal

using System;

class HexadecimalToDecimal
{
    static void Main()
    {
        string n = Console.ReadLine();
        long dec = 0;

        for (int i = 0; i < n.Length; i++)
        {
            switch (n[i])
            {
                case 'A': dec += 10 * (long)Math.Pow(16, n.Length - i - 1); break;
                case 'B': dec += 11 * (long)Math.Pow(16, n.Length - i - 1); break;
                case 'C': dec += 12 * (long)Math.Pow(16, n.Length - i - 1); break;
                case 'D': dec += 13 * (long)Math.Pow(16, n.Length - i - 1); break;
                case 'E': dec += 14 * (long)Math.Pow(16, n.Length - i - 1); break;
                case 'F': dec += 15 * (long)Math.Pow(16, n.Length - i - 1); break;
                default: dec += (long)char.GetNumericValue(n[i]) * (long)Math.Pow(16, n.Length - i - 1); break;
            }
        }

        Console.WriteLine(dec);
    }
}