N = gets.to_i
B = gets.split(" ").map{|s| s.to_i}

from = []
to = []

cnt = 0
B.each_with_index {|n, i|
	if n == 0 then
		if from.length > 0
			f = from.shift
			cnt += i - f
		else
			to << i
		end
	else 
		while n > 1 and to.length > 0 do
			t = to.shift
			cnt += i - t
			n -= 1
		end
		if n > 1 then
			from.concat([i] * (n-1))
		end
	end
}

puts cnt