#include <bits/stdc++.h>
using namespace std;

int main(void){
    int n,m;
    cin >> n >> m;
    vector<int> a(n+2);
    for(int i=1;i<=n;++i){
        cin >> a[i];
    }
    vector<int> b(n+2);
    for(int i=1;i<=n;++i){
        cin >> b[i];
        b[i]-=a[i];
        b[i]+=m;
        b[i]%=m;
    }
    int cnt=0;
    vector<int> diff(n+1);
    for(int i=0;i<=n;++i){
        diff[i]=b[i+1]-b[i];
        if(diff[i]<0){
            ++cnt;
        }
        diff[i]+=m;
        diff[i]%=m;
    }
    sort(diff.begin(),diff.end(),greater<int>());
    long long ans=0;
    for(int i=0;i<cnt;++i){
        ans+=m-diff[i];
    }
    cout << ans << endl;
    return 0;
}