#include <bits/stdc++.h>
using namespace std;

int main(){
    int N;
    int64_t L, X[100000], Y[100000];
    cin >> N >> L;
    for(int i=0; i<N; i++) cin >> X[i];
    for(int i=0; i<N; i++) cin >> Y[i];

    vector<pair<int64_t, int64_t>> order;
    for(int i=0; i<N; i++){
        order.push_back({X[i], 1});
        order.push_back({Y[i], -1});
    }
    sort(order.begin(), order.end());

    int mn = 0, now = 0;
    for(auto& p : order){
        now += p.second;
        mn = min(mn, now);
    }
    mn *= -1;

    order.clear();
    for(int i=0; i<N; i++){
        order.push_back({X[i], 1});
        if(i < mn){
            order.push_back({Y[i]+L, -1});
        }else{
            order.push_back({Y[i], -1});
        }
    }
    sort(order.begin(), order.end());
    int64_t ans = 0;
    now = 0;
    for(auto& p : order){
        if(p.second == 1){
            now++;
        }else{
            now--;
            ans = max(ans, now*L + p.first);
        }
    }
    cout << ans << endl;
    return 0;
}