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

#define rep(i, m, n) for(int(i) = (int)(m); i < (int)(n); ++i)
#define rep2(i, m, n) for(int(i) = (int)(n)-1; i >= (int)(m); --i)
#define REP(i, n) rep(i, 0, n)
#define REP2(i, n) rep2(i, 0, n)
#define all(hoge) (hoge).begin(), (hoge).end()
#define en '\n'
using ll = long long;
using ull = unsigned long long;
template <class T>
using vec = vector<T>;
template <class T>
using vvec = vector<vec<T>>;
typedef pair<ll, ll> P;
using tp = tuple<ll, ll, ll>;
constexpr long long INF = 1LL << 60;
constexpr int INF_INT = 1 << 25;
//constexpr long long MOD = (ll)1e9 + 7;
constexpr long long MOD = 998244353LL;
using ld = long double;
static const ld pi = 3.141592653589793L;
typedef vector<ll> Array;
typedef vector<Array> Matrix;

template <class T>
inline bool chmin(T &a, T b) {
    if(a > b) {
        a = b;
        return true;
    }
    return false;
}
template <class T>
inline bool chmax(T &a, T b) {
    if(a < b) {
        a = b;
        return true;
    }
    return false;
}

//グラフ関連
struct Edge {
    ll to, cap, rev;
    Edge(ll _to, ll _cap, ll _rev) {
        to = _to;
        cap = _cap;
        rev = _rev;
    }
};
typedef vector<Edge> Edges;
typedef vector<Edges> Graph;

void add_edge(Graph &G, ll from, ll to, ll cap, bool revFlag, ll revCap) {
    G[from].push_back(Edge(to, cap, (ll)G[to].size()));
    if(revFlag)
        G[to].push_back(Edge(from, revCap, (ll)G[from].size() - 1));
}

void solve() {
    ll n, l;
    cin >> n >> l;
    vec<ll> x(n);
    vec<ll> y(n);

    set<ll> stx, sty;

    REP(i, n) {
        cin >> x[i];
        stx.insert(x[i]);
    }

    REP(i, n) {
        cin >> y[i];
        sty.insert(y[i]);
    }

    auto it = stx.begin();
    ll ans = 0;
    ll t = 0;
    while(stx.size()) {
        //次のお茶
        auto sy = sty.upper_bound(*it);
        if(sy == sty.end()) {
            sy = sty.begin();
            t++;
        }

        //次のお茶より後にある寿司
        auto tx = stx.upper_bound(*sy);
        if(tx == stx.end()) {
            tx = stx.begin();
            t++;
        }

        //効率を落とさずに最後に飲めるお茶
        auto ty = sty.upper_bound(*tx);
        if(ty == sty.begin())
            ty = sty.end();
        ty--;

        if(stx.size() == 1) {
            cout << l * (t - 1) + *ty << en;
            return;
        }

        sty.erase(ty); //最後のお茶をのむ
        if(*it == *tx) {
            it = stx.erase(it);
        } else {
            stx.erase(it);
            it = tx; //次の寿司
        }
    }
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
    /*
    ll t;
    cin >> t;
    while(t--)*/
    solve();

    return 0;
}