n = gets.to_i
a = Array.new(n-1)
(n-1).times do |i|
    a[i] = gets.to_i
end
cnt = 0
ans = 0
(n-1).times do |i|
    b, c = gets.split.map(&:to_i)
    cnt -= b
    cnt += c
    ans += a[i] * cnt
end
p ans