n = int(input())
a = [int(input()) for _ in range(n-1)]
b = []
c = []
for _ in range(n):
    bi, ci = map(int, input().split())
    b.append(bi)
    c.append(ci)

current_passengers = 0
total = 0

for i in range(n):
    current_passengers -= b[i]
    current_passengers += c[i]
    if i + 1 < n:
        total += a[i] * current_passengers

print(total)