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

B=[]
if A[0]!=1:
  B.append(A[0]-1)
for i in range(M-1):
  B.append(A[i+1]-A[i]-1)
if A[-1]!=N:
  B.append(N-A[-1])

def calc(n):
  if n==1:
    return 0
  elif n%2==1:
    return 2*calc((n-1)//2)+1
  else:
    return 0

ans=sum(b-calc(b) for b in B)
print(ans)