#!/usr/bin/env ruby

require 'zipf'

ng = [{},{},{},{}]

while line = STDIN.gets
  ngrams(line, 4) { |g|
    if ng[g.size-1].has_key? g
      ng[g.size-1][g] += 1
    else
      ng[g.size-1][g] = 1
    end
  }
end

rr = 1.0
ng.each_with_index { |h,j|
  singletons = ng[j].reject { |k,v| v > 1 }.size
  rr *= (ng[j].size - singletons).to_f/ng[j].size.to_f
}

rr = rr**0.25

puts rr