import heapq
import sys
input = sys.stdin.readline
N, M = map(int, input().split())
A = list(map(int, input().split()))
ans = 0
dp = []
for i in range(M-1):
    heapq.heappush(dp, (-(A[i+1]-A[i]), A[i], A[i+1]))

while dp:
    c, a, b = heapq.heappop(dp)
    if (-c)==2:
        ans += 1
    elif (-c)%2==0:
        m = (a+b)//2
        heapq.heappush(dp, (-(m-a), a, m))
        heapq.heappush(dp, (-(b-m), m, b))
    else:
        ans += b-a-1
print(ans)