using System;
using System.Globalization;
using System.Linq;
using System.Collections.Generic;
 
public class Test
{
 
 
	public static void Main()
	{
		string s="aa bb cc dd ee ff gg hh ii kk";
        var fifthBlankIndex = s.Select((c, index) => new{ c, index })
        .Where(x => Char.IsWhiteSpace(x.c))
        .ElementAtOrDefault(4);
 
        int result = -1;
        if(fifthBlankIndex  != null)
            result = fifthBlankIndex.index;
        Console.WriteLine(result);
	}
}

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