#include<bits/stdc++.h>
using namespace std;
using LL=long long;
using ULL=unsigned long long;
#define rep(i,n) for(int i=0; i<(n); i++)

int N;
LL K;
LL A[100000],B[100000];
LL D[100000];

int main(){
	cin>>N>>K;
	rep(i,N) cin>>A[i];
	rep(i,N) cin>>B[i];
	D[0]=0;
	if(N>=2) D[1]=A[0]+B[1];
	for(int i=2; i<N; i++) D[i]=min(D[i-1]+A[i-1]+B[i],D[i-2]+A[i-2]+B[i]+K);
	LL ans=0; rep(i,N) ans=max(ans,D[i]);
	cout<<ans<<endl;
	return 0;
}