Related problem: 100299K - Digraphs

My code: 290508668

The first case in test 3 was (like the following one):

676
aa
ab
ac
...

And my output for the case was (ASCII 96):

`

That was an illegal character, yet the checker gave me an accepted???

It's been 10 (or 11) years yet nobody have discovered this before...

MikeMirzayanov fix this please...