N,M = map(int,input().split())
A = list(map(int,input().split()))
C = 10 ** 6
dp = [0] * C
dp[1] = 1
dp[2] = 2
for i in range(3,C):
	if i  % 2 == 1:
		dp[i] = dp[i//2] * 2
	else:
		dp[i] = dp[i // 2] * 2 + dp[i // 2 - 1] * 2

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