PreprimeNumbers | Topcoder Archive

Problem Statement

A number is preprime if it has exactly 4 positive integer divisors. For example, 6 is preprime because its divisors are 1, 2, 3, and 6. The integers 6, 8, 10, 14 form the beginning of an infinite sequence of preprime numbers. Find the n-th element of this sequence, where n is a 1-based index.

Definition

Class:: PreprimeNumbers
Method:: nthNumber
Parameters:: int
Returns:: int
Method signature:: int nthNumber(int n)
(be sure your method is public)

Constraints

n will be between 1 and 1000000, inclusive.

Examples

```
2
```
```
Returns: 8
```
The beginning of an infinite sequence of preprime numbers is: 6, 8, 10, 14, ... The second number is 8.
```
4
```
```
Returns: 14
```
The beginning of an infinite sequence of preprime numbers is: 6, 8, 10, 14, ... The fourth number is 14.
```
24
```
```
Returns: 77
```
```
43765
```
```
Returns: 193539
```

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