For example if I take N =4. then i write:
10 12
9 3
15 10
11 13

What will I get?

i think that no such input like yours. ( my ac solution does not handle it properly. )

you should write it in this way (only one digit for one number per line):
1 1
0 2
9 3
1 1
5 0
1 1
1 3
and therefore N = 7