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)