N = gets.to_i
B = gets.split.map(&:to_i)

stock = []
empty = []
ans = 0

B.each_with_index do |b, idx|
  if b == 0
    if stock.empty?
      empty << idx
    else
      i, cnt = stock.pop

      ans += idx - i
      cnt -= 1

      stock.push([i, cnt]) if cnt > 0
    end
  else
    while empty.size > 0 && b > 0
      i = empty.pop
      ans += idx - i
      b -= 1
    end

    if b == 0
      empty.push(idx)
    elsif b >= 2
      stock.push([idx, b - 1])
    end
  end
end

puts ans