This is very important:
"SMS message which consists of latin letters and spaces only can be up to M characters long while the length of SMS message which consists of any characters is limited by N characters."
Because non-latin letters are coded by another charset and message header will took 5 extra bytes. Read the problem task about N and M you will understad;
test case:
3    5
aaaa      答案是 1

3 5
aaa!       答案是 2

http://blog.csdn.net/zhangyanxing666/article/details/9831623