def main():
    n = int(input())
    a = [int(input()) for _ in range(n-1)]
    b = []
    c = []
    for _ in range(n):
        b_, c_ = map(int, input().split())
        b.append(b_)
        c.append(c_)

    current = 0
    fee_sum = 0

    for fee, exit, enter in zip(a, b, c):
        current += (enter-exit)
        fee_sum += current * fee
    print(fee_sum)


if __name__ == "__main__":
    main()