#include<iostream>
#include<set>
using namespace std;
typedef long long ll;

int main(){
    int n, l;
    cin >> n >> l;
    set<int> s[2];
    int in;
    for(int i = 0; i < n; i++)  cin >> in, s[0].insert(in);
    for(int i = 0; i < n; i++)  cin >> in, s[1].insert(in);
    ll ans = 0;
    int take = *(s[0].begin()), turn = 0;
    s[0].erase(s[0].begin());
    while(!s[0].empty() || !s[1].empty()){
        auto its = s[turn].upper_bound(take);
        auto itt = s[1-turn].upper_bound(take);
        if(its == s[turn].end())    its = s[turn].begin();
        if(itt == s[1-turn].end())  itt = s[1-turn].begin();
        int su = *its, te = *itt;
        if(take > su)   su += l;
        if(take > te)   te += l;
        if(te > su){
            int ntake = *itt;
            s[1-turn].erase(itt);
            if(take > ntake)    ans += l;
            take = ntake;
        }else{
            auto it = s[1-turn].lower_bound(*its);
            if(it == s[1-turn].begin()) it = s[1-turn].end();
            it--;
            int ntake = *it;
            s[1-turn].erase(it);
            if(take > ntake)    ans += l;
            take = ntake;
        }
        turn = 1-turn;
    }
    cout << ans+take << endl;
    return 0;
}