'''
8の時
4 1 3
oooo-o*o
3 1 4
o*o-oooo

'''
N,M=map(int,input().split())
A=list(map(int,input().split()))
ans=0
for i in range(1,M):
    tmp=A[i]-A[i-1]-1
    count=1
    while tmp!=1 and tmp%2!=0:
        tmp//=2
        count*=2
    ans+=count*tmp
print(ans)