#include <bits/stdc++.h>

using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

typedef int _loop_int;
#define REP(i,n) for(_loop_int i=0;i<(_loop_int)(n);++i)
#define FOR(i,a,b) for(_loop_int i=(_loop_int)(a);i<(_loop_int)(b);++i)
#define FORR(i,a,b) for(_loop_int i=(_loop_int)(b)-1;i>=(_loop_int)(a);--i)

#define DEBUG(x) cout<<#x<<": "<<x<<endl
#define DEBUG_VEC(v) cout<<#v<<":";REP(i,v.size())cout<<" "<<v[i];cout<<endl
#define ALL(a) (a).begin(),(a).end()

#define CHMIN(a,b) a=min((a),(b))
#define CHMAX(a,b) a=max((a),(b))

// mod
const ll MOD = 1000000007ll;
#define FIX(a) ((a)%MOD+MOD)%MOD

// floating
typedef double Real;
const Real EPS = 1e-11;
#define EQ0(x) (abs(x)<EPS)
#define EQ(a,b) (abs(a-b)<EPS)
typedef complex<Real> P;

int n,l;
int x[125252], y[125252];

int main(){
  scanf("%d%d",&n,&l);
  REP(i,n)scanf("%d",x+i);
  REP(i,n)scanf("%d",y+i);
  set<int> S,T;
  REP(i,n)S.insert(x[i]);
  REP(i,n)T.insert(y[i]);
  ll ans = 0;
  int turn = 0;
  while(true){
    // try
    int scnt = 0, ocnt = 0;
    int pos = 0;
    int tt = turn;
    while(tt < 2*n){
      if(tt%2==0){
        auto it = S.lower_bound(pos);
        if(it==S.end())break;
        tt++; scnt++; pos = *it;
      }else{
        auto it = T.lower_bound(pos);
        if(it==T.end())break;
        tt++; ocnt++; pos = *it;
      }
    }
    if(tt == 2*n){
      ans += pos; break;
    }
    ans += l;
    pos = l;
    turn = tt;
    while(scnt+ocnt>0){
      tt--;
      if(tt%2==0){
        auto it = S.lower_bound(pos); it--;
        pos = *it; S.erase(it);
        scnt--;
      }else{
        auto it = T.lower_bound(pos); it--;
        pos = *it; T.erase(it);
        ocnt--;
      }
    }
  }
  printf("%lld\n",ans);
  return 0;
}