n = int(input())
a = [int(input()) for _ in range(n-1)]
bcs = [tuple(map(int, input().split())) for _ in range(n)]

current_passengers = 0
total = 0

for i in range(1, n):
    # Process bus stop i (i ranges from 1 to n-1)
    b, c = bcs[i-1]
    current_passengers -= b  # passengers get off
    current_passengers += c  # passengers get on
    total += current_passengers * a[i-1]  # travel to next stop

print(total)