Problem Link: http://www.codechef.com/problems/RRMTRX2/

I read the whole problem 3-4 times but unable to understand that why there is an interference of negative numbers during calculations.

Also i didn’t understand the output details

Output single integer — the answer for the problem modulo 107 + 7, i.e the smallest non-negative integer number r that answer - r is divisible by 107 + 7.

Can anyone please explain me, i think so i have not understood the problem or i guess i am missing some important line mentioned in problem.

Please Help…!!

In the constraints there is a line

```
0 ≤ |Ai, j| ≤ 100
```

This means that the elements in the array can be negative numbers also.

i.e., `-100 <= A(i,j) <= +100`

That why there is an interference of negative numbers during calculations.

Now to answer your second doubt, as there can be negative elements, their multiplications can be negative.

So A(i,j)*A(x,y) can be both negative or positive (maybe even zero).

You have to output the resultant sum in a modulo 10^7 +7 but as there exists negative numbers thee result of modulo can also be negative. You need to show the non-negative part of it.

That means if resultant sum is -12 and MOD is 5 then, -12 % 5 = -2, -2+5=3, this is the non negative ans.

Now " the smallest non-negative integer number r that answer - r is divisible by 107 + 7. " means in our example,

ans=3; 3-r => 3-(-12)=15 and 15%5 = 0.

Hope this helps.

1 Like

okay thanks that’s what i was missing, absolute values…!!

Thanks…

Your welcome.

The problem statement was difficult to understand in the beginning for me too .

Could not understand the MOD part of the output and it cost me 2 WAs during the contest :(.

yahh thats true, felt same…

Problem statement was hard but its really interesting…!