N,M = map(int,raw_input().split(" "))
D = [-1000000000] + [input() for i in range(M)] + [10000000000000]

pos = 0
ans = 0
for i in range(N):
    a,p,pos = min([(abs(pos-D[i]),i,D[i]) for i in range(len(D))])
    ans += a
    D.pop(p)

print ans