#define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;
#define FOR(i,j,k) for(int (i)=(j);(i)<(int)(k);++(i))
#define rep(i,j) FOR(i,0,j)
#define each(x,y) for(auto &(x):(y))
#define mp make_pair
#define MT make_tuple
#define all(x) (x).begin(),(x).end()
#define debug(x) cout<<#x<<": "<<(x)<<endl
#define smax(x,y) (x)=max((x),(y))
#define smin(x,y) (x)=min((x),(y))
#define MEM(x,y) memset((x),(y),sizeof (x))
#define sz(x) (int)(x).size()
#define RT return
using ll = long long;
using pii = pair<int, int>;
using vi = vector<int>;
using vll = vector<ll>;

void solve() {
    int N, L;
    cin >> N >> L;
    set<int> S[2];
    rep(i, 2) {
        rep(j, N) {
            int x;
            cin >> x;
            S[i].insert(x);
            S[i].insert(x + L);
        }
        S[i].insert((int)2e9 + 9);
    }

    ll ans = 0;
    int pos = 0;
    rep(i, 2 * N - 1) {
        int mod = i % 2;
        int x = *S[mod].lower_bound(pos);
        int y = *S[!mod].lower_bound(x);
        int z = *--S[mod].lower_bound(y);
        ans += z - pos;
        pos = z % L;
        S[mod].erase(pos);
        S[mod].erase(pos + L);
    }
    ans += *S[1].lower_bound(pos) - pos;
    cout << ans << endl;
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(0);
	cout << fixed << setprecision(15);
	solve();
	return 0;
}