n=int(input())
p=list(map(int,input().split()))
c=0
for i in range(n):
    if i!=n-1
        if p[i]==i+1:
            d=p[i+1]
            p[i+1]=p[i]
            p[i]=d
            c+=1
    else:
        if p[i]==i+1:
            c+=1
print(c)