#include <iostream>
#include <algorithm>
#include <iomanip>
#include <vector>
#include <queue>
#include <set>
#include <map>

using namespace std;
typedef long long ll;

ll N, L;
ll x[100000];
ll y[100000];

int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout << setprecision(10) << fixed;
    ll cur = 0;
    ll ans = 0;
    set<ll> stx, sty;
    cin >> N >> L;
    for(int i = 0; i < N; i++){
        cin >> x[i]; stx.insert(x[i]);
    }
    for(int i = 0; i < N; i++){
        cin >> y[i]; sty.insert(y[i]);
    }
    cur = x[0];
    for(int i = 0; i < N; i++){
        // sushi
        auto p = sty.upper_bound(cur);
        if(p == sty.end()){
            p = sty.upper_bound(0);
        }
        auto q = stx.upper_bound(*p);
        if(q == stx.begin()){
            q = stx.end();
        }
        q--;
        if(i == 0) ans += (*q+L)%L;
        else ans += (*q-cur+L)%L;
        cur = *q;
        stx.erase(*q);
        // cout << *q << ' ';
        // tea
        p = stx.upper_bound(cur);
        if(p == stx.end()){
            p = stx.upper_bound(0);
        }
        q = sty.upper_bound(*p);
        if(q == sty.begin()){
            q = sty.end();
        }
        q--;
        ans += (*q-cur+L)%L;
        cur = *q;
        sty.erase(*q);
        // cout << *q << endl;
    }
    cout << ans << endl;
}