N = gets.to_i
A = Array.new(N + 1, 0)
(2 .. N).each{|i| A[i] = gets.to_i }
pay = 0
(1 .. N).inject(0) do |s, i|
  pay += s * A[i]
  b,c = gets.split.map(&:to_i)
  s - b + c
end
puts pay