def num_of_area( line )
	n1 = 2
	n2 = 2 
	2.upto( line ){|n|
		n2 = n1 + n	
		n1 = n2
	}
	return n2
end

answers = Array.new
while line = $stdin.gets
	n = line.chomp.to_i
	answers << num_of_area( n )
end
answers.each{ |a| puts a }
