N = int(input())
A = [0]
B = []
C = []
for i in range(N-1):
    A.append(int(input()))
for i in range(N):
    b, c = map(int, input().split())
    B.append(b)
    C.append(c)

people = 0
fare = 0
for i in range(N):
    fare += people * A[i]
    people += C[i] - B[i]

print(fare)