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

p = [i for i in range(N)]

s = set()

for v in S:
    v -= 1

    l,r = p[v],p[v+1]
    x,y = min(l,r),max(l,r)
    s.add( (x,y) )
    p[v],p[v+1] = r,l

print (len(s))