import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################

def f(x):
    if x == 0:
        return 0
    if x == 1:
        return 1
    if x % 2 == 0:
        return x
    return f((x-1)//2) * 2

n, m = na()
a = na()
ans = 0
for i in range(m-1):
    ans += f(a[i+1] - a[i] - 1)

print(ans)