size = ARGV[0].to_i # NOTE: ARGV[0] is _not_ the program/script name....

myset1 = Set.new((0..size).map {|i| "x#{i*1000}"} )
myset2 = Set.new((0..size).map {|i| "x#{i*1000+1}"} )

(0..10000).each do
  res = myset1 - myset2
end