def solution(a)
  start_points = a.each_with_index.map{|x,i| i-x }.sort
  end_points = a.each_with_index.map{|x,i| i+x}.sort

  intersections = 0
  open_intervals = 0

  while !start_points.empty? && !end_points.empty? do
    if end_points.first < start_points.first
      end_points.shift
      open_intervals -= 1
    else
      intersections += open_intervals
      open_intervals += 1
      start_points.shift
    end
  end

  intersections > 10 * 1000 * 1000 ? -1 : intersections
end