#include <bits/stdc++.h>
using namespace std;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) reps(i,0,n)
#define Rreps(i,n,e) for(int i = n - 1; i >= e; --i)
#define Rrep(i,n) Rreps(i,n,0)
#define ALL(a) a.begin(), a.end()
#define fi first
#define se second
typedef long long ll;
typedef vector<ll> vec;
typedef vector<vec> mat;

ll N,M,H,W,Q,K,A,B;
string S;
//const ll MOD = 998244353;
const ll MOD = (1e+9) + 7;
typedef pair<ll, ll> P;
const ll INF = (1LL<<58);


int main() {
    cin>>N>>K;
    vec a(N), b(N);
    rep(i,N) cin>>a[i];
    rep(i,N) cin>>b[i];
    vec dp(N, 0);
    if(N != 1) dp[1] = dp[0] + a[0] + b[1];
    reps(i, 2, N){
        dp[i] = min(dp[i-1] + a[i-1] + b[i], dp[i-2] + a[i-2] + b[i] + K);
    }
    cout<<*max_element(ALL(dp))<<endl;
}