def cnt(x):
	if x == 1 or x % 2 == 0:
		return x
	return 2*cnt(x//2)

N, M = map(int, input().split())
A = list(map(int, input().split()))
ans = 0
for i in range(M-1):
	ans += cnt(A[i+1]-A[i]-1)
print(ans)