import sys
input = sys.stdin.readline
N = int(input())
road = list(map(int, input().split()))
oil = list(map(int, input().split()))
res = road[0] * oil[0]
min = oil[0]
for i in range(1, N-1):
if min > oil[i]:
min = oil[i]
res += min * road[i]
print(res)

서브태스트 문제는 처음이었는데 신기하당