#include <bits/stdc++.h>
using namespace std;
int MOD = 1e9 + 7;
long long int power(long long int a, long long int b) {
  long long int res = 1;
  a = a % MOD;
  while (b > 0) {
    if (b & 1) {
      res = (res * a) % MOD;
      b--;
    }
    a = (a * a) % MOD;
    b >>= 1;
  }
  return res;
}
long long int fermat_inv(long long int y) { return power(y, MOD - 2); }
long long int gcd(long long int a, long long int b) {
  return (b == 0) ? a : gcd(b, a % b);
}
long long int min(long long int a, long long int b) { return (a > b) ? b : a; }
long long int max(long long int a, long long int b) { return (a > b) ? a : b; }
bool prime[1000001];
vector<int> primes;
void SieveOfEratosthenes(int n) {
  memset(prime, true, sizeof(prime));
  for (int p = 2; p * p <= n; p++) {
    if (prime[p] == true) {
      for (int i = p * p; i <= n; i += p) prime[i] = false;
    }
  }
  for (int p = 2; p < 1000001; p++)
    if (prime[p]) primes.push_back(p);
}
long long int fact[1001];
void factorial(int n) {
  fact[0] = 1;
  for (int i = 1; i <= n; i++) fact[i] = fact[i - 1] * i, fact[i] %= MOD;
}
long long int ncr(long long int n, long long int r) {
  if (n < r)
    return -1;
  else {
    long long int x = fact[r] * fact[n - r] % MOD;
    return fact[n] * fermat_inv(x) % MOD;
  }
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  int t = 1;
  while (t--) {
    int n, m;
    cin >> n >> m;
    long long int h[n], p[m];
    for (int i = 0; i < n; i++) cin >> h[i];
    for (int i = 0; i < m; i++) cin >> p[i];
    long long int ans = LLONG_MAX;
    long long int l = 0, r = 1e18;
    while (l <= r) {
      long long int mid = (l + r) / 2;
      long long int j = 0, k = -1;
      bool b = true;
      for (int i = 0; i < n; i++) {
        if (p[j] < h[i] - mid) {
          b = false;
          break;
        }
        if (p[j] >= h[i])
          k = h[i] + mid;
        else
          k = h[i] + max(mid - 2 * (h[i] - p[j]), (mid - (h[i] - p[j])) / 2);
        while (j < m && p[j] <= k) j++;
        if (j == m) break;
      }
      if (j == m) {
        ans = min(ans, mid);
        r = mid - 1;
      } else
        l = mid + 1;
    }
    cout << ans << '\n';
  }
}
