I got AC with 0.015s,216Kb
I had read the problem several time until understand.
N is the length of the digit number.
It will be something like that
a1 a2 a3 ...an
for each ai in a1 a2 a3..an, 0 <= ai < K.
The output number i used was interger (compiler C++ 2010). (no big number).

hope it can help you.
And sorry for my poor English.

Edited by author 25.04.2014 23:05

thanks an phan. through your post, i have understanded a bit. but why K = 10. when a1 a2 a3....an <10 and lager or equal 0. i thinks result = 99. why 90? i suspect it? Thanks you so muck.

From task:
> 0001235 is not a 7-digit number, it is a 4-digit number.

So first zero isn't allowed. So,
0 < a1 < 10
0 <= a2...an < 10

Result is 9*10 = 90

Maybe you just thought "00" is not the valid number, but the problem said "0001235 is not a 7-digit number, it is a 4-digit number. ", that means  01, 02...09 are not 2-digit numbers, they are 1-digit numbers, so the answer should be 90.

Dear anphan . Your question is jam <3 <3<3