## Problem J5: Unfriend

Mark invited some people to join his social network. Some of them invited new people, who invited new people, and so on. Now there are N people in the network, numbered from 1 to N. Mark has decided to remove some people and keep others. There is one restriction: when removing a person, he will also remove the people s/he invited, and the people they invited, and so on. Mark will never remove himself, and we do not allow people to be invited by more than one person. Mark can also decide to not remove anyone.

How many different sets of people can be removed?

### Input Format

The first line contains a single integer N (N ≤ 6), the number of people in the network. Next are N − 1 lines telling us who invited each person. To be precise, line i in this set (1 ≤ i ≤ N − 1) contains a single integer j (j > i), which indicates that person j is the person who invited person i. Person N is Mark.

### Output Format

Output a single integer, the number of possible sets of people that can be removed.

```3
3
3```

`4`

### Explanation

The first number of the input indicates there are three people in the network. The next line tells us that Person 1 was invited by Mark, while the last line tells us that Person 2 was also invited by Mark. The sets of people that can be removed are {}, {1}, {2}, {1,2}.

```4
3
4
4```

`6`

### Explanation

There are 4 people in the network. Here is a table of who invited whom:

 Person inviting Invited 1 none 2 none 3 1 4 2,3

The possible sets are {}, {1}, {2}, {1,2}, {1,3}, and {1,2,3}. Notice that the sets {3} and {2,3} are not psosible, since when you remove 3, you must also remove 1.

Point Value: 7
Time Limit: 2.00s
Memory Limit: 16M