N = gets.to_i
A = (N - 1).times.map { gets.to_i }
Q = N.times.map { gets.split.map(&:to_i) }

ans = 0
cur = 0

Q.each.with_index do |(b, c), i|
  ans += cur * A[i - 1] if i > 0

  cur += (c - b)
end

puts ans