n = int(input())
A = []
B = []
for i in range(n):
    A.append(int(input()))
for i in range(n+1):
    B.append(input().split())
    
ride = 0
kane = 0

for i in range(n):
    ride += int(B[i][1])
    ride -= int(B[i][0])
    kane += ride*A[i]
    
print(kane)