import sys
input = sys.stdin.readline

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