N = int(input())
p=list(map(int, input().split()))
#前と後ろどっちと交換すればよいか
ans=0
for i in range(N):
    if i<N-1:
        if p[i]==i+1:
            a=p[i]
            b=p[i+1]
            p[i]=b
            p[i+1]=a
            ans+=1
    if i==N-1:
        if p[i]==i+1:
            a=p[i]
            b=p[i-1]
            p[i]=b
            p[i+1]=a
            ans+=1
print(ans)
