N, M = map(int,input().split())
D = [int(input()) for _ in range(M)]

C = 0
P = 0
while N > 0:
    L = []
    for i in D:
        L.append(abs(P - i))
    
    X = min(L)
    Y = L.index(X)
    P = D.pop(Y)
    N -= 1
    C += X

print(C)