(n, m), a = $<.map{ _1.split.map &:to_i }
x = 0
puts [(1..m).map { |i|
  x += a[-1] - a[-i]
  m.to_f / i + 1.0 / i * x
}.min, a[-1] - a[0]].min + (a[0] - 1) + (n - a[-1])