The error message means that you need to invoke volteado and siprimo on an instance of the Program class. E.g.:
...
Program p = new Program();
long av = p.volteado(a); // av is "a" but swapped
if (p.siprimo(a) == false && p.siprimo(av) == false)
...
They cannot be invoked directly from the Main method because Main is static while volteado and siprimo are not.
The easiest way to fix this is to make the volteado and siprimo methods static:
private static bool siprimo(long a)
{
...
}
private static bool volteado(long a)
{
...
}
