n = int(input())
fare = []
passenger = 0
pay = 0

for i in range(n-1):
    fare.append(int(input()))

out,come = map(int,input().split())
passenger += come

for i in range(n-1):
    pay += passenger * fare[i]
    out,come = map(int,input().split())
    passenger = passenger - out + come

print(pay)