Stirling number of the second kind

Stirling number of the second kind

#include <iostream>

size_t S(size_t n, size_t k)
{
   if(k > n)
   {
      return 0;
   }
   else if(k == 1)
   {
      return 1; 
   }
   else
   {
      return S(n-1, k-1) + k * S(n-1, k);
   }
}

int main()
{
   std::cout << S(6,4) << std::endl;
   return 0;
}