N = gets.not_nil!.to_i

P = [0] of Int32
B = [] of Int32
C = [] of Int32

(N-1).times do
  P << P[-1] + gets.not_nil!.to_i
end

N.times do
  b, c = gets.not_nil!.split.map(&.to_i)
  B << b
  C << c
end

D = Array(Int32).new(N, 0)
ans = 0

N.times do |i|
  b = B[i]
  (0...i).each do |j|
    break if b == 0
    d = [D[j], b].min
    ans += d * (P[i] - P[j])
    D[j] -= d
    b -= d
  end
  D[i] += C[i]
end

puts ans