N, M = gets.split.map(&:to_i)
A = gets.split.map(&:to_i)
W = Array.new(N) { [] }
Q = []
max_w = 0

M.times do
  x, w = gets.split.map(&:to_i)
  x -= 1

  max_w = w if max_w < w
  Q << [x, w]
  W[x] << w
end

def f(c)
  cnt = 0
  weight = 0
  sum = Array.new(N, 0)
  counter = Array.new(N + 1, 0)
  remain = Array.new(N + 1, 0)

  0.upto(N - 1) do |i|
    cnt -= counter[i]
    weight -= remain[i]

    weight -= c * cnt
    sum[i] += weight

    W[i].each do |w|
      dist = (c == 0) ? N : w / c
      next if dist == 0

      weight += w
      n_idx = (i + 1) + dist
      n_idx = N if n_idx > N
      counter[n_idx] += 1
      remain[n_idx] += w % c if c > 0
      cnt += 1
    end
  end

  cnt = 0
  weight = 0
  counter = Array.new(N + 1, 0)
  remain = Array.new(N + 1, 0)

  (N - 1).downto(0) do |i|
    cnt -= counter[i]
    weight -= remain[i]

    weight -= c * cnt
    sum[i] += weight

    W[i].each do |w|
      dist = (c == 0) ? -1 : w / c
      next if dist == 0

      n_idx = (i - 1) - dist
      weight += w
      cnt += 1

      if n_idx >= 0
        counter[n_idx] += 1
        remain[n_idx] += w % c if c > 0
      end
    end
  end

  Q.each do |x, w|
    sum[x] += w
  end

  (0..N - 1).all? { |i| sum[i] < A[i] }
end

if f(0)
  puts 0
  exit
elsif !f(max_w)
  puts -1
  exit
else
  ok = max_w
  ng = 0

  while (ok - ng).abs >= 2
    c = (ok + ng) / 2

    if f(c)
      ok = c
    else
      ng = c
    end
  end

  puts ok
end