N, M = gets.split.map(&:to_i)
E = Hash.new { |h, k| h[k] = [] }

P = []

M.times do
  a, b = gets.split.map(&:to_i)

  E[a] << b
  P << a
  P << b
end

P.unshift(1)
P.push(N)
P.uniq!
P.sort!
L = P.size
dp = Array.new(L, Float::INFINITY)
dp[0] = 0
I = Hash.new

P.each_with_index do |pos, idx|
  I[pos] = idx
end

(L - 1).times do |i|
  a = P[i]
  n_pos = P[i + 1]
  dist = 2 * (n_pos - a)

  dp[i + 1] = dp[i] + dist if dp[i] + dist < dp[i + 1]

  E[a].each do |u|
    idx = I[u]
    b = P[idx]
    dist = 2 * b - 2 * a - 1

    dp[idx] = dp[i] + dist if dp[i] + dist < dp[idx]
  end
end

puts dp.last