n = int(input())
p = list(map(int, input().split()))
cnt = 0
for i in range(n-1):
  if p[i] == i+1:
    p[i],p[i+1] = p[i+1],p[i]
    cnt += 1
  elif p[i+1] == i+2:
    p[i],p[i+1] = p[i+1],p[i]
    cnt += 1
print (cnt)