node = int(input())
road_list = list(map(int,input().split()))
price_list = list(map(int,input().split()))
total_price = 0
now_price = price_list[0]
for i in range(len(road_list)):
total_price+=road_list[i]*now_price
if now_price > price_list[i+1]:
now_price = price_list[i+1]
print(total_price)