#include <bits/stdc++.h>
using namespace std;
const int maxn = 100000 + 5;
const long long INF = 100000000000000;
long long h[maxn], p[maxn];
long long l[maxn], r[maxn];
int n, m;
bool ok(long long t) {
  for (int i = 0; i < n; i++) {
    l[i] = h[i];
    r[i] = h[i];
  }
  int k = 0;
  for (int i = 0; i < m; i++) {
    if (p[i] >= l[k] && p[i] <= r[k]) continue;
    if (p[i] < l[k]) {
      if (r[k] - p[i] > t) {
        return false;
      } else {
        l[k] = p[i];
      }
    }
    if (p[i] > r[k]) {
      if (p[i] - l[k] + min(p[i] - h[k], h[k] - l[k]) > t) {
        k++;
        if (k >= n) return false;
        i--;
      } else {
        r[k] = p[i];
      }
    }
  }
  return true;
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 0; i < n; i++) scanf("%I64d", &h[i]);
  for (int i = 0; i < m; i++) scanf("%I64d", &p[i]);
  long long l = -1, r = INF;
  while (l < r - 1) {
    long long mid = (l + r) / 2;
    if (ok(mid)) {
      r = mid;
    } else {
      l = mid;
    }
  }
  cout << r << endl;
}
