#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, int>> order;
    for(int i=0; i<N; i++){
        order.push_back({X[i], -1});
        if(Y[i] < X[0]){
            order.push_back({Y[i]+L, 1});
        }else{
            order.push_back({Y[i], 1});
        }
    }
    sort(order.rbegin(), order.rend());

    int64_t now = 0, mx = 0, pos = 0;
    for(auto& p : order){
        now += p.second;
        if(now < 0) now++;
        if(now > mx){
            mx = now;
            pos = p.first;
        }
    }
    int64_t ans = (mx-1) * L + pos;
    cout << ans << endl;
    return 0;
}