n,m=map(int,input().split())
s=list(map(int,input().split()))
a=list(range(n))
p=[]
for v in s:
	p+=[(min(a[v-1],a[v]),max(a[v-1],a[v]))]
	a[v-1],a[v]=a[v],a[v-1]
print(len(set(p)))