合成数列の和	https://qiita.com/advent-calendar/2018/composite-number
Python(/Ruby)	https://qiita.com/cielavenir/items/dcbd839b0c66af1bc971
Crystal	https://qiita.com/cielavenir/items/1a0dd1a94aca9935f5aa
Kuin	https://qiita.com/cielavenir/items/47fd18fc68055d990599
Swift	https://qiita.com/cielavenir/items/6cde3b20866c842dc9e2
Kotlin	https://qiita.com/cielavenir/items/f58eaa30f90781229a6d
Pascal	https://qiita.com/cielavenir/items/35c2fc54ca16b620c9c7

Ruby版の移植ですね

composite.cr

def isqrt(n)
	return 0 if n<=0
	return 1 if n<4 # 1
	x,y=0,n
	while x!=y&&x+1!=y
		x,y=y,(n/y+y)/2
	end
	x
end
p (4..1/0.0).each.select{|i|(2..isqrt(i)).any?{|j|i%j<1}}.first(gets.not_nil!.to_i).sum