I think that problem means that leading zeroes may be added, if one number is shorter than the other. But I'm sure that there is a test, where both numbers have leading zeroes. For example, smth like that:
4
0 0
0 0
9 0
5 1
I printed 96, but this is wrong. Correct output is 0096. Now I got AC.

And, of course, may be situation like that:
5
0 0
9 0
9 0
9 0
9 2
Correct answer is 10001.

But why do you think it's a bug?

Quote from problem description:
"Output file should contain exactly N digits"
So, output "96" is wrong, but "0096" is right.

The problem is not in wrong output, but in test.
"N(1<=N<=1000000) - the length of the integers (in order to make their lengths equal, some leading zeroes can be added)."
It means that this test is impossible and incorrect, because the length of 0095 is 2, the length of 0001 is 1. And because of leading zeroes are required only to make the length equal, correct input must be
2
9 0
5 1

thank you very very much. i found my bug.