N,M=map(int, input().split())
A=list(map(int, input().split()))
D=[i for i in range(N)]
B=[]
for a in A:
  x,y=D[a-1],D[a]
  B.append((min(x,y),max(x,y)))
  D[a],D[a-1]=D[a-1],D[a]
  
print(len(set(B)))