#############################################################

import sys
sys.setrecursionlimit(10**7)

from heapq import heappop,heappush
from collections import deque,defaultdict,Counter
from bisect import bisect_left, bisect_right
from itertools import product,combinations,permutations

ipt = sys.stdin.readline

def iin():
    return int(ipt())
def lmin():
    return list(map(int,ipt().split()))

#############################################################

N,M = lmin()
A = [0]+lmin()

ans = 0
for i in range(M):
    if (A[i+1]-A[i]-1)%2 and A[i+1]-A[i]-1 >= 3:
        ans += A[i+1]-A[i]-2
    else:
        ans += A[i+1]-A[i]-1

print(ans)