#include <bits/stdc++.h>
using namespace std;
struct fast_ios { fast_ios(){ cin.tie(0); ios::sync_with_stdio(false); cout << fixed << setprecision(20); }; } fast_ios_;
#define FOR(i, begin, end) for(int i=(begin);i<(end);i++)
#define REP(i, n) FOR(i,0,n)
#define IFOR(i, begin, end) for(int i=(end)-1;i>=(begin);i--)
#define IREP(i, n) IFOR(i,0,n)
#define Sort(v) sort(v.begin(), v.end())
#define Reverse(v) reverse(v.begin(), v.end())
#define all(v) v.begin(),v.end()
#define SZ(v) ((int)v.size())
#define Lower_bound(v, x) distance(v.begin(), lower_bound(v.begin(), v.end(), x))
#define Upper_bound(v, x) distance(v.begin(), upper_bound(v.begin(), v.end(), x))
#define Max(a, b) a = max(a, b)
#define Min(a, b) a = min(a, b)
#define bit(n) (1LL<<(n))
#define bit_exist(x, n) ((x >> n) & 1)
#define Ans(f, y, n) if(f) cout << y << endl; else cout << n << endl;
#define debug(x) cout << #x << "=" << x << endl;
#define vdebug(v) cout << #v << "=" << endl; REP(i, v.size()){ cout << v[i] << ","; } cout << endl;
#define mdebug(m) cout << #m << "=" << endl; REP(i, m.size()){ REP(j, m[i].size()){ cout << m[i][j] << ","; } cout << endl;}
#define pb push_back
#define f first
#define s second
#define int long long
#define INF 1000000000000000000

using vec = vector<int>;
using mat = vector<vec>;
using Pii = pair<int, int>;
using PiP = pair<int, Pii>;
using PPi = pair<Pii, int>;
using bools = vector<bool>;
using pairs = vector<Pii>;

template<typename T> void readv(vector<T> &a){ REP(i, a.size()) cin >> a[i]; }
void readv_m1(vector<int> &a){ REP(i, a.size()){cin >> a[i]; a[i]--;} }

//int dx[4] = {1,0,-1,0};
//int dy[4] = {0,1,0,-1};

int mod = 1000000007;
//int mod = 998244353;
#define Add(x, y) x = (x + (y)) % mod
#define Mult(x, y) x = (x * (y)) % mod



signed main(){

    int N, L; cin >> N >> L;
    vec x(N); readv(x);
    vec y(N); readv(y);

    set<int> X, Y;
    for(int i: x){
        X.insert(i);
        X.insert(i + L);
    }
    for(int i: y){
        Y.insert(i);
        Y.insert(i + L);
    }
    int ans = 0;
    int l = 0;
    REP(i, N){
        auto it = X.lower_bound(l);
        int l2 = *it;
        ans += l2 - l;
        X.erase(l2 % L); X.erase(l2 % L + L);
        l = l2;
        if(l >= L) l -= L;

        bool e = false;
        if(i < N - 1){
            auto next = X.lower_bound(l);
            it = Y.lower_bound(*next);
            if(it != Y.begin()){
                it--;
                if(*it >= l) e = true;
            }
        }

        if(!e) it = Y.lower_bound(l);
        l2 = *it;
        ans += l2 - l;
        Y.erase(l2 % L); Y.erase(l2 % L + L);
        l = l2;
        if(l >= L) l -= L;
    }

    cout << ans << endl;

    return 0;
}