# require "template"
lib C
  fun strtoll(s : UInt8*, p : UInt8**, b : Int32) : Int64
end

class String
  def to_i64
    C.strtoll(self, nil, 10)
  end
end

n, m = read_line.split.map(&.to_i)
a = read_line.split.map(&.to_i64)
last = 0i64
puts [(1..m).min_of { |i|
  last += a[-1] - a[-i]
  m / i + 1 / i * last
}, a[-1] - a[0]].min + (a[0] - 1) + (n - a[-1])