#include<bits/stdc++.h>
using namespace std;
int n,c,r[100001],a[100001],s,t;
int main(){
    scanf("%d%d",&n,&c);
    for(int i=1;i<=n;i++) scanf("%d",&r[i]);
    for(int i=1;i<=n;i++){
        scanf("%d",&a[i]);
        if(a[i]>=r[i]){a[i]-=r[i]; t+=a[i];}
    }
    s=t*c;
    printf("%d",s);
    return 0;
}