using System;
using System.Collections;
using System.Linq;

public class Test
{
    public static void Main()
	{
        var str="2017991";
        var n=2;
        Console.WriteLine( 
            String.Join("/",
                Enumerable.Range(0, (str.Length+n-1) / n -1 )
               // .AsParallel()
                .Select(i => str.Substring(i * n, n))
                .ToArray()
            )
        );		
	}
}