a.rb

require 'json'

input = "[[0,5,6,11,15],[12,24,63],[17,28,50]]"

input = JSON.parse(input)

start = Time.now
min_column = input.map{|a| a[0]}
max = min_column.max
second_smallest = min_column.sort[1]
min = input[min_column.index(min_column.min)].max_by do |x|
  if x > second_smallest
    x = -1
  end
  x
end

puts "Range: #{min} - #{max}"
puts (Time.now - start).to_f