#include <bits/stdc++.h>
#include <atcoder/segtree>
using namespace atcoder;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000000000000000

array<int,2> op(array<int,2> a,array<int,2> b){
	rep(i,2)a[i] = max(a[i],b[i]);
	return a;
}

array<int,2> e(){
	return array<int,2>{-1,-1};
}

int main(){

	int N;
	cin>>N;
	
	long long L;
	cin>>L;
	
	vector<pair<long long,int>> V(2*N);
	rep(i,N){
		scanf("%lld",&V[i].first);
		V[i].second = 0;
	}
	
	rep(i,N){
		scanf("%lld",&V[i+N].first);
		V[i+N].second = 1;
	}
	
	sort(V.begin(),V.end());
	
	vector<array<int,2>> y(4*N,e());
	rep(i,2*N){
		y[i][V[i].second] = i;
		y[i+2*N][V[i].second] = 2*N+i;
	}
	
	segtree<array<int,2>,op,e> seg(y);
	
	vector<int> ans;
	int now = 0;
	
	while(ans.size()!=2*N){
		int temp = 0;
		if(ans.size()>0)temp = ans.back();
		
		int ok = 4*N,ng = temp;
		while(ok-ng>1){
			int mid = (ok+ng)/2;
			array<int,2> ret = seg.prod(temp,mid);
			if(ret[now]==-1)ng = mid;
			else ok = mid;
		}
		int x;
		{
			array<int,2> ret = seg.prod(temp,ok);
			x = ret[now^1];
		}
		ok = temp,ng = 4*N;
		while(ng-ok>1){
			int mid = (ok+ng)/2;
			array<int,2> ret = seg.prod(temp,mid);
			if(ret[now^1]<=x)ok = mid;
			else ng = mid;
		}
		
		array<int,2> ret = seg.prod(temp,ok);
		x = ret[now]%(2*N);
		ans.push_back(x);
		seg.set(x,e());
		seg.set(x+2*N,e());
		
		now ^= 1;
	}
	
	
	long long Ans = 0LL;
	
	rep(i,2*N){
		if(i==0)Ans += V[ans[i]].first;
		else{
			long long temp = V[ans[i]].first - V[ans[i-1]].first;
			if(temp<0)temp += L;
			Ans += temp;
		}
	}
	
	cout<<Ans<<endl;
		
	
	return 0;
}