PROGRAMMING:Set partition

When n = 4, the set {1,2,3,4} can be divided into 15 different nonempty subsets as follows:
```
{{1}，{2}，{3}，{4}}，
{{1，2}，{3}，{4}}，
{{1，3}，{2}，{4}}，
{{1，4}，{2}，{3}}，
{{2，3}，{1}，{4}}，
{{2，4}，{1}，{3}}，
{{3，4}，{1}，{2}}，
{{1，2}，{3，4}}，
{{1，3}，{2，4}}，
{{1，4}，{2，3}}，
{{1，2，3}，{4}}，
{{1，2，4}，{3}}，
{{1，3，4}，{2}}，
{{2，3，4}，{1}}，
{{1，2，3，4}}
```
How many nonempty subsets can the set of n elements {1,2,..., n} be divided into?
###Input format:
A positive integer n
###Output format:
A number that represents the number of nonempty subsets that can be divided by the set of n elements
###Input example:
```in
five
```
###Output example:
```out
fifty-two
```






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