title: Nth Pentagonal Number
signature: nthPentagonalNumber :: Integer -> Double
  nthPentagonalNumber n = 
source_py: def nth_pentagonal_number(n):
      return ((((3 * n) * n) - n) / 2)
source_cpp: int nth_pentagonal_number(int n) {
    return ( 3 * n * n - n ) / 2;
  }