I saw some explanations, it's rare, but the example is:
10 15
On the 11-th of February, 2006 the contest "Timus Top Coders: First Challenge" is held!

Some explanations have this:
On the 11-         <1>
th of February     <2>
, 2006 the         <3>
 contest "         <4>
Timus Top Coders   <5>
:                  <6>
 First Challenge   <7>
" is held!         <8>
I think the second is wrong, they are 14 caracters and we have M = 15, and N = 10, so we cannot have this. .. please any suggesttion, may be I din't understand something.

The length of 2nd line cannot be 15, because it'll include ',', which will force you to send only 10 chars. So it's better to send only 14 chars in 2nd message.

Your message don't have to be exactly of length of 10 or 15.