Monkey and river

def solution(a, d)
  return 0 if a.size < d

  length = a.size
  position = 0

  (1..(length * 2)).each do |time|
    allowed_raduis_data = a[position..([position+d-1, length].min)]
    max_jump_position = allowed_raduis_data.reverse.find_index{|el| el != -1 && (el-time) <= 0}
    next if max_jump_position.nil?

    position += allowed_raduis_data.size - max_jump_position

    return [time, length, a.max].min if length - position < d
  end

  -1
end