from bisect import bisect_left

N,M = map(int,input().split())
INF = 10**18
A = [-INF]+list(map(int,input().split()))+[INF]
B = list(map(int,input().split()))

C = []
i = -1
ans = 0
c = 0
b_cp = 0

def plus_num():
  global ans
  ans += A[i]-A[i-1]-max(c,A[i]-b_cp)

for b in B:
  bis = bisect_left(A,b)
  if bis == i:
    C.append(b)
    c = max(c,C[-1]-C[-2])
  else:
    if C:
      plus_num()
      C = [b]
    i = bis
    c = 0
    C.append(b)
  b_cp = b
if C:
  plus_num()
print(ans)