n=int(input())
alist = [int(i) for i in input().split()]
ans=0
 
for i in range(n-1):
	if alist[i]==i+1:
		temp=alist[i+1]
		alist[i+1]=alist[i]
		alist[i]=temp
		ans+=1
if alist[n-1]=n:
	ans+=1
print(ans)