N,M = map(int,input().split())
A = list(map(int,input().split()))

def calc(n):
	if n == 1:return 1
	elif n == 2:return 2
	else:
		if n % 2 == 1:
			return 2 * calc(n // 2)
		else:
			return 2 * (calc(n // 2) + calc(n //2 - 1))
ans = 0
for i in range(M - 1):
	ans += calc(A[i + 1] - A[i])