#include<iostream>
#include<string>
#include<algorithm>
#include<vector>
#include<iomanip>
#include<math.h>
#include<complex>
#include<queue>
#include<deque>
#include<stack>
#include<map>
#include<set>
#include<bitset>
#include<functional>
#include<assert.h>
#include<numeric>
using namespace std;
#define REP(i,m,n) for(int i=(int)(m) ; i < (int) (n) ; ++i )
#define rep(i,n) REP(i,0,n)
typedef long long ll;
typedef pair<int,int> pint;
typedef pair<ll,int> pli;
const int inf=1e9+7;
const ll longinf=1LL<<60 ;
const ll mod=1e9+7 ;

int main(){
    int n;
    cin>>n;
    ll T;
    cin>>T;
    set<ll> sushi,tea;
    rep(i,n){
        ll x;
        cin>>x;
        sushi.insert(x);
        sushi.insert(T+x);
        sushi.insert(2*T+x);
    }
    rep(i,n){
        ll x;
        cin>>x;
        tea.insert(x);
        tea.insert(T+x);
        tea.insert(2*T+x);
    }
    ll cur=0;
    int cnt=0;
    ll ans=0;
    while(1){
        auto itr=sushi.lower_bound(cur);
        ans+=*itr-cur;
        ll ret=*itr;
        if(ret>T)ret-=T;
        sushi.erase(ret);
        sushi.erase(ret+T);
        sushi.erase(ret+2*T);
        ll res=*tea.lower_bound(ret);
        if(cnt==n-1){
            ans+=res-ret;
            break;
        }
        ll nxt=*sushi.lower_bound(res);
        res=*prev(tea.lower_bound(nxt));
        ans+=res-ret;
        while(res>T)res-=T;
        tea.erase(res);
        tea.erase(res+T);
        tea.erase(res+2*T);
        cur=res;
        ++cnt;
    }
    cout<<ans<<endl;
    return 0;
}