N, M = gets.split.map(&:to_i)

memo = Array.new(M + 1) { Array.new(N + 2, Float::INFINITY) }

1.upto(N * M) do |t|
  n, m = gets.split.map(&:to_i)

  if t <= 5
    t = 0
  else
    t -= 5
  end

  memo[m][n] = t if memo[m][n] > t
  if n == 1
    memo[m][N + 1] = t if memo[m][N + 1] > t
  end
end

ans = Float::INFINITY

1.upto(M) do |m|
  1.upto(N - 4) do |n|
    cards = memo[m][n..n + 4].sort
    draw_cnt = 5
    cur = 0
    cnt = 0

    while cards.size > 0
      while cards.size > 0 && cards[0] <= cur
        cards.shift
        draw_cnt -= 1
      end

      if draw_cnt > 0
        next_i = cards[0]
        len = next_i - cur
        c = (len + draw_cnt - 1) / draw_cnt
        cnt += c
        cur += c * draw_cnt
      end
    end

    ans = cnt if ans > cnt
  end
end

puts ans