def soln(L):
  if L%2 == 0: return L
  if L == 1: return 1
  return 2*soln((L-1)//2)

N, M = map(int, input().split())
A = list(map(int, input().split()))

ans = 0
for i in range(M-1):
  ans += soln(A[i+1]-A[i]-1)

print(ans)