n,m = map(int,input().split())
a = list(map(int,input().split()))
p = list(map(int,input().split()))
i = 0
j = 0
min_p = 1001
remain_candy = 0
sum_money = 0
while i<len(p):
    if p[i]<min_p:
        min_p = p[i]
        sum_a_day = (sum(a[j:i])-remain_candy+1)//m
        sum_money += sum_a_day * p[i-1]
        remain_candy = sum_a_day * m - sum(a[j:i])
        j = i
        i = i+1
    else:
        i += 1
    if i==len(p):
        sum_a_day = (sum(a[j:])-remain_candy+1)//m
        sum_money += sum_a_day * p[j]
        break
print(sum_money)