Data

#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
#define ll long long 
const int maxn=1e5+10;
int n,m;
ll a[maxn],b[maxn];
bool check(ll mid)
{
    for(ll i=1,j=1;i<=n;i++)
    {
        if(a[i]-b[j]>mid) return false;//如果从i到j的距离比假设的答案大，就返回0继续向右二分
        ll reach;//reach表示走完mid步之后到达的位置 
        if(a[i]>b[j])
        {
            ll rest=mid-(a[i]-b[j]);
            reach=max(b[j]+rest,a[i]+rest/2);//向左边走和向右边走 
        }
        else
            reach=a[i]+mid;
        while(b[j]<=reach&&j<=m) j++;
        if(j>m) return true; 
    }
    return false;
}
void solve()
{
    ll l=0,r=1e10,ans;
    while(l<=r)
    {
        ll mid=(l+r)/2;
        if(check(mid)) r=mid-1,ans=mid;
        else l=mid+1;
    }
    printf("%I64d",ans);
}
int main()
{
    freopen("data.in","r",stdin);
    freopen("data.out","w",stdout);
    scanf("%d%d",&n,&m);
    for(int i=1;i<=n;i++)
        scanf("%I64d",&a[i]);
    for(int i=1;i<=m;i++)
        scanf("%I64d",&b[i]);
    /*int flag=0;
    if(n==m)
    {
        for(int i=1;i<=n;i++)
            if(a[i]!=b[i])
                flag=1;
    }
    if(flag==0&&n==m)
        printf("0");
    else
    {*/
        solve();
    //}
    fclose(stdin);fclose(stdout);
    return 0;
}