using System;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var x = string.Concat(

                "A;B;C;D;E;"

                .Split(";")

                .Select((letter, i) => letter + (i % 2 == 0 ? ">" : ";")))

                .Trim(';', '>');

​

            Console.WriteLine(x);

    }

}