we dont know use the locomotive as a carriage, like:
input:
1 1
AA
AB

output:
NO

GOOD LUCK!!!

Thank you! I got WA#14 but it's in past. =)

Thanks too! I've got WA#14 in the past and I've got AC now.

It means, be careful that the end should be the same as the locomotive.

e.g. if the locomotive is "AA" then the last should be "AA" or "BA", such as "ABBA" with locomotive "AA" is correct but "ABBA" with locomotive "BA" is incorrect.
(sorry for my bad English)

Thanks!
I got WA on Test #14
After reading this,I know my understanding was wrong