>>336はミス。0がこんなに多いわけがない

require 'bigdecimal'
[3, 5, 7].each{|i|
n = 1000*i - 1
puts "N = %i => %i"%[i, BigDecimal(i).sqrt(n).floor(n).to_s(?F).count(?0)]
}

N = 3 => 309
N = 5 => 492
N = 7 => 738