
def main():
    n = int(input())
    p = list(map(int,input().split()))


    ans =0
    for i in range(n-1):
        if p[i] == i+1:
            tmp = p[i]
            p[i] = p[i+1]
            p[i+1] = tmp
            ans+=1

    print(ans)

if __name__ == "__main__":
        main()
