For any number larger than 2, the primes reached when you start with n and concatenate the sum of its prime factors with its largest prime factor, then repeat the process until you eventually reach a prime, or print a -1 if you never do.

A125674

For any number larger than 2, the primes reached when you start with n and concatenate the sum of its prime factors with its largest prime factor, then repeat the process until you eventually reach a prime, or print a -1 if you never do.