#include <iostream>
#include <cstdio>
#include <cmath>
#include <ctime>
#include <cstdlib>
#include <cassert>
#include <vector>
#include <list>
#include <stack>
#include <queue>
#include <deque>
#include <map>
#include <set>
#include <bitset>
#include <string>
#include <algorithm>
#include <utility>
#define llint long long
#define inf 1e18
#define rep(x, s, t) for(llint (x) = (s); (x) < (t); (x)++)
#define Rep(x, s, t) for(llint (x) = (s); (x) <= (t); (x)++)
#define chmin(x, y) (x) = min((x), (y))
#define chmax(x, y) (x) = max((x), (y))
#define mod 1000000007

using namespace std;
typedef pair<llint, llint> P;

struct edge{
	llint to, cost;
	edge(){}
	edge(llint a, llint b){
		to = a, cost = b;
	}
};

llint n, k;
llint a[100005], b[100005];
vector<edge> G[100005];
llint dist[100005];

void dijkstra(vector<edge> G[], llint S, llint dist[])
{
	for(int i = 1; i <= n; i++) dist[i] = inf;
	dist[S] = 0;
	
	priority_queue< P, vector<P>, greater<P> > Q;
	Q.push( make_pair(0, S) );
	
	llint v, d;
	while(Q.size()){
		d = Q.top().first;
		v = Q.top().second;
		Q.pop();
		if(dist[v] < d) continue;
		for(int i = 0; i < G[v].size(); i++){
			if(dist[G[v][i].to] > d + G[v][i].cost){
				dist[G[v][i].to] = d + G[v][i].cost;
				Q.push( make_pair(dist[G[v][i].to], G[v][i].to) );
			}
		}
	}
}

int main(void)
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	cin >> n >> k;
	for(int i = 1; i <= n; i++) cin >> a[i];
	for(int i = 1; i <= n; i++) cin >> b[i];
	
	for(int i = 2; i <= n; i++){
		if(i-2 >= 1) G[i-2].push_back(edge(i, a[i-2]+b[i]+k));
		G[i-1].push_back(edge(i, a[i-1]+b[i]));
	}
	dijkstra(G, 1, dist);
	
	llint ans = 0;
	for(int i = 1; i <= n; i++) ans = max(ans, dist[i]);
	cout << ans << endl;
	
	return 0;
}