Request for help with ordering integers


New Drupal user here! I am using Drupal to post articles for a magazine on my website. The articles are grouped by issue number. I have just tried to upload the articles which comprise issue 10, but they are appearing between issues 1 and 2 in the numerically sorted list. I want issue 10 to appear after issue 9.

I have tried changing 1 and 2 to 01 and 02, but when I try to save changes they revert to 1 and 2.

Any help would be much appreciated, I am still very much a novice!