#include <bits/stdc++.h>
using namespace std;
const int maxn = 100007;
int n, m;
long long h[maxn], p[maxn];
bool check(long long time) {
  int x = 1, old_x;
  for (int i = 1, _c = n; i <= _c; i++) {
    if (x > m) return 1;
    if (p[x] <= h[i]) {
      if (h[i] - p[x] > time) return 0;
      old_x = x;
      while (x <= m && h[i] >= p[x]) x++;
      if (old_x != x) {
        while (x <= m &&
               p[x] - p[old_x] + min(p[x] - h[i], h[i] - p[old_x]) <= time)
          x++;
      }
    } else {
      while (x <= m && p[x] - h[i] <= time) x++;
    }
  }
  return (x > m);
}
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1, _c = n; i <= _c; i++) scanf("%I64d", &h[i]);
  for (int i = 1, _c = m; i <= _c; i++) scanf("%I64d", &p[i]);
  long long l, r, mid, res;
  r = res = 20000000000ll;
  l = 0;
  while (l <= r) {
    mid = (l + r) / 2;
    if (check(mid)) {
      res = mid;
      r = mid - 1;
    } else
      l = mid + 1;
  }
  cout << res << endl;
  return 0;
}
