Convert Excel Range To Absolute Excel Range, C#

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// excel address to absolute address conversion
// supply a sheet name and a range, and outputs an absolute range

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text.RegularExpressions;

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            //Your code goes here
            Console.WriteLine(ConvertToFullAddress("Report","A1:B2"));
        }
        
        
        
        static string[] SplitCellAddr(string celladdr) {
            string letters="";
            string nums="";
            int mode=0;
            for(int k=0; k<celladdr.Length; k++) {
                if(Char.IsDigit(celladdr[k])) {
                    nums+=celladdr[k];
                } else if(Char.IsLetter(celladdr[k])) {
                    letters+=celladdr[k];
                }
            }
            return new string[] { letters, nums };
        }
        public static string ConvertToFullAddress(string sheetname, string inputaddr) {
            string[] pair=inputaddr.Split(':');
            string[] firstcell = SplitCellAddr(pair[0]);
            string[] secondcell = SplitCellAddr(pair[1]);
            return "'"+sheetname+"'!$"+firstcell[0]+"$"+firstcell[1]+":$"+secondcell[0]+"$"+secondcell[1];
        }
        
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,12 sec, absolute running time: 0,09 sec, cpu time: 0,09 sec, average memory usage: 14 Mb, average nr of threads: 3