N = int(input())
p = list(map(int, input().split()))

count = 0
for i in range(N):
  if p[i] == i+1:
    p[i], p[i+1] = p[i+1], p[i]
    count += 1
if p[N-1] == N:
  p[N-2], p[N-1] = p[N-1], p[N-2]
  count += 1
  
print(count)