Serial number based on different initial codes upon user

[Mantenimiento_S]

Hello everyone. First of all I want to thank you for your help. 
I have a field where I write down a consecutive serial number. 
For each user this serial number starts with a different number. Example:
User 1: 242501001
User 2: 242502001
User 3: 242503001
How can I do so that, depending on the user and their specific serial number, I can add a consecutive number?
For example:
242501001 +1
242502001 +1
Thanks in advance