# help in modular arithmetic for FLOORI4

I have checked my code for several times for different situations… But I think the error is in the modular part! Can anyone please help me with that please!!!

http://www.codechef.com/viewsolution/4860500

Hi, you need to find ( (n*(n+1)*(2*n+1)*(3*n*n+3*n-1))/30 )% m right ? you can follow as below :

1. First multiply ‘m’ by ‘30’ and say it as M.i.e M=m*30
2. Now just perform normal modulo using M. i.e using the same trick as (a*b)%c=((a%c)*(b%c))%c,take care of overflows
3. Let the result of modulo be x; now just divide x by 30 and that’s your final answer

You can refer POWER() method in my solution for more clarity : [http://www.codechef.com/viewsolution/4803010][1]

For proof of this technique refer : [http://discuss.codechef.com/questions/50953/floori4-editorial/51016][2]
[1]: http://www.codechef.com/viewsolution/4803010
[2]: http://discuss.codechef.com/questions/50953/floori4-editorial/51016

4 Likes

You need to multiply m by 30 ie M=30*m

@chalubhalu Oh sorry that’s a typo mistake and thanks for the correction

@achaitanyasai Thankyou very much for replying but I am doing the same thing with modulus… I am getting correct answer for some cases but wrong answer for some cases like for n=12 and m=10… Please can you help me with that! I have checked and re-checked my code!