I have wa#2 for many times, but at last I have done it.
If you are in same situation, notice M can be larger than N. You should output something - the largest number in 1..N.

It's can't be.

Another reason could be data-type not able to hold values in the range of 0 and 100000

M is always less or equal to N man, read the problem once more for assurance and also i don't think that WA can be cause of data-type which can't hold values from 0 to 10^6..simple "integer/int" is ok man.

test case 2 has exactly 25000 measured values....
eg-
3
10
11
1
19
...
...
...
(25000 integers)
-1


so,be careful in declaring the size of array... :)(you can use arr[25001] ).