n, m = list(map(int,input().split()))
t = list(map(int, input().split()))
li = [k for k in range(n)]
ptli = [None for i in range(m)]
for j in range(m):
  li[t[j]-1], li[t[j]] = li[t[j]], li[t[j]-1]
  ptli[j] = tuple(sorted([li[t[j]-1], li[t[j]]]))
print(len(set(ptli)))