#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <cmath>
#include <map>
#include <queue>
#include <iomanip>
#include <set>
#include <tuple>
#define mkp make_pair
#define mkt make_tuple
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define all(v) v.begin(),v.end()
using namespace std;
typedef unsigned long long ll;
const ll MOD=1e9+7;
template<class T> void chmin(T &a,const T &b){if(a>b) a=b;}
template<class T> void chmax(T &a,const T &b){if(a<b) a=b;}

const ll INF=1e19;

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);

  int N;
  ll K;
  cin>>N>>K;
  vector<ll> A(N),B(N);
  rep(i,N) cin>>A[i];
  rep(i,N) cin>>B[i];

  vector<ll> dp(N,INF);
  dp[0]=0;
  for(int i=1;i<N;i++){
    if(i-1>=0) chmin(dp[i],dp[i-1]+A[i-1]+B[i]);
    if(i-2>=0) chmin(dp[i],dp[i-2]+A[i-2]+B[i]+K);
  }

  ll ans=dp[0];
  rep(i,N) chmax(ans,dp[i]);
  cout<<ans<<endl;

  return 0;
}