PROGRAMMING:Take the stairs

As we all know, $$Keven $$likes to take stairs, and for some reasons, $$Keven $$can only take one or two stairs at a time. Now, $$Keven $$has $$n $$stairs to go. Now, $$Keven $$wants to know how many different ways he can go to the $$n $$stairs, because $$Keven $$has to climb stairs, so this question is up to you.
###Input format:
The first line is an integer $$n $$, meaning as above
$$(1<=n<=40)$$
###Output format:
Output an integer in one line to indicate how many different ways $$Keven $$can go to the $$n $$stairs
###Input example:
```in
three
```
###Output example:
```out
three
```







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