import sys
input = sys.stdin.readline
import bisect

N,M=map(int,input().split())
A=[-1<<60]+list(map(int,input().split()))+[1<<60]
B=list(map(int,input().split()))

X=[[] for i in range(len(A))]

for b in B:
    x=bisect.bisect(A,b)
    X[x-1].append(b)

#print(X)

ANS=0

for i in range(len(A)-1):
    if X[i]==[]:
        continue

    else:
        X2=[A[i]]+X[i]+[A[i+1]]

        P=X2[-1]-X2[0]

        MAX=-1
        for i in range(len(X2)-1):
            MAX=max(MAX,X2[i+1]-X2[i])

        ANS+=P-MAX

print(ANS)