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

c = []
d = 0
for i in b:
    d +=  i[1] - i[0]
    c.append(d)

e = 0
for i in range(len(a)):
    e += a[i] * c[i]
print(e)