class Input_kyopro:
    def II(self): return int(input())
    def MI(self): return map( int,input().split())
    def MS(self): return map(str,input().split())
    def LMI(self): return list(self.MI())
    def LMS(self): return list(self.MS())
    def LLI(self,N): return [self.LMI() for _ in range(N)]
    def LLS(self,N): return [self.LMS() for _ in range(N)]
    def LS(self,N): return [input() for _ in range(N)]
    def LSL(self,N): return [list(input()) for _ in range(N)]
    def LI(self,N): return [self.II() for _ in range(N)]
I=Input_kyopro()
#入力
N,M=I.MI()
A=I.LMI()
sa=[]
for i in range(1,M):
    if A[i]-A[i-1]-1!=0:
        sa.append(A[i]-A[i-1]-1)
ans=0
for a in sa:
    if a%2==0:
        ans+=a
    else:
        if a!=1:
            ans+=a-1
        else:
            ans+=1
print(ans)