using System;

public class Test
{
        private const int NumbersOfLettersInAlphabet = 26;

        public static void Main()
        {
            Console.WriteLine(ExcelNumerationToDecimal("AB"));

        }

        public static int ExcelNumerationToDecimal(string columnName)
        {
            var columnNumber = 0;
            for (var i = 0; i < columnName.Length; i++)
            {
                columnNumber += (columnName[columnName.Length - i - 1] - '@') * (int)Math.Pow(NumbersOfLettersInAlphabet, i);
            }
            return columnNumber;
        }
}