def calc(num):
    if num==0:
        return 0
    if num==1:
        return 1
    if num%2==0:
        return num
    return calc(num//2)*2

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