#include<bits/stdc++.h>
using namespace std;
long long a[1000005];
long long b[1000005];
int main(){
	long long n,m,sum = 0,s = 0,qian = 0,c = 0;
	cin>>n>>m;
	for(long long i = 1;i<n;i++){
		cin>>a[i];
	}
	for(long long i = 1;i<=n;i++){
		cin>>b[i];
	}
	for(long long i = 1;i<=n;i++){
		if(i==n) break;
		for(long long j = i+1;j<=n;j++){
			if(b[j]<b[i]){
				for(long long x = i;x<=j-1;x++) s+=a[x];
				long long aa = s-sum;
				aa = aa+m-1;
				aa/=m;
				qian+=aa*b[i];
				sum+=aa*m;
				c = 1;
				i = j-1;
				break;
			}
		}
		if(c==0){
			for(long long x = i;x<=n-1;x++) s+=a[x];
			long long aa = s-sum;
			aa = aa+m-1;
			aa/=m;
			qian+=aa*b[i];
			sum+=aa*m;
		}
		c = 0;
	}
	cout<<qian;
	return 0;
}