#include <bits/stdc++.h>
#define int ll
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)
#define per(i,n) for(int i=n-1;i>=0;i--)
#define rng(i,c,n) for(int i=c;i<n;i++)
#define fi first
#define se second
#define pb push_back
#define sz(a) (int)a.size()
#define vec(...) vector<__VA_ARGS__>
#define _4aNWZeC ios::sync_with_stdio(0),cin.tie(0)
using ll=long long;
using pii=pair<int,int>;
using vi=vector<int>;
void print(){cout<<'\n';}
template<class h,class...t>
void print(const h&v,const t&...u){cout<<v<<' ',print(u...);}

void slv(){
	int n,m;
	cin>>n>>m;
	vi a(n);
	rep(i,n){
		cin>>a[i];
	}
	vi b(n);
	rep(i,n){
		cin>>b[i];
	}
	vi d(n);
	rep(i,n){
		d[i]=(a[i]-b[i]+m)%m;
	}
	vi nd;
	nd.pb(d[0]);
	rng(i,1,n){
		nd.pb((d[i]-d[i-1]+m)%m);
	}
	nd.pb((2*m-d.back())%m);
	d=nd;
	n+=1;
	int ans=0;
	deque<int> dq(d.begin(), d.end());
	sort(dq.begin(), dq.end());
	while(1){
		while(sz(dq) and dq.front()==0) dq.pop_front();
		while(sz(dq) and dq.back()==m) dq.pop_back();
		if(!sz(dq)) break;
		assert(sz(dq)>1);
		int val=min(dq[0],m-dq.back());
		dq[0]-=val;
		dq.back()+=val;
		ans+=val;
	}
	cout<<ans<<"\n";
}

signed main(){
_4aNWZeC;
	slv();
}