#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 p[252521];
int sum[252521];

int main(){
  scanf("%d%d",&n,&l);
  REP(i,n)scanf("%d",x+i);
  REP(i,n)scanf("%d",y+i);
  REP(i,n)p[i] = 2*x[i]+0;
  REP(i,n)p[i+n] = 2*y[i]+1;
  sort(p,p+n+n);
  REP(i,2*n)sum[i+1] = sum[i]+(p[i]%2==0 ? 1 : -1);
  int s=0, t=0;
  REP(i,2*n+1){
    if(sum[i]>=sum[s])s=i;
    if(sum[i]<sum[t])t=i;
  }
  ll ans = (ll)l * (max(0,sum[s]-1) + (-sum[t]));
  if(sum[s] > -sum[t] || (sum[s]==-sum[t] && s>t)){
    s--;
    int id = -1;
    FOR(i,s+1,2*n)if(p[i]%2==1){
      id = i; break;
    }
    if(id==-1){
      ans += l;
      REP(i,s)if(p[i]%2==1){
        id = i; break;
      }
    }
    ans += p[id]/2;
    printf("%lld\n",ans);
  }else{
    t--;
    ans += p[t]/2;
    printf("%lld\n",ans);
  }
  return 0;
}