#include <bits/stdc++.h>
using namespace std;
int main(){
  int N, M;
  cin >> N >> M;
  vector<int> A(N);
  for (int i = 0; i < N; i++){
    cin >> A[i];
  }
  vector<int> B(M);
  for (int i = 0; i < M; i++){
    cin >> B[i];
  }
  vector<vector<int>> C(N + 1);
  for (int i = 0; i < M; i++){
    int p = lower_bound(A.begin(), A.end(), B[i]) - A.begin();
    C[p].push_back(B[i]);
  }
  int ans = 0;
  if (!C[0].empty()){
    ans += A[0] - C[0][0];
  }
  if (!C[N].empty()){
    ans += C[N][C[N].size() - 1] - A[N - 1];
  }
  for (int i = 1; i < N; i++){
    if (!C[i].empty()){
      int cnt = C[i].size();
      int D = A[i] - A[i - 1];
      int mx = 0;
      mx = max(mx, C[i][0] - A[i - 1]);
      mx = max(mx, A[i] - C[i][cnt - 1]);
      for (int j = 0; j < cnt - 1; j++){
        mx = max(mx, C[i][j + 1] - C[i][j]);
      }
      ans += D - mx;
    }
  }
  cout << ans << endl;
}