Problem:
Score : 300 points

Problem Statement

You are given integers N and K. Find the number of triples (a,b,c) of positive integers not greater than N such that a+b,b+c and c+a are all multiples of K.
The order of a,b,c does matter, and some of them can be the same.

Constraints

1 \leq N,K \leq 2\times 10^5

N and K are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

Print the number of triples (a,b,c) of positive integers not greater than N such that a+b,b+c and c+a are all multiples of K.

Sample Input 1

3 2

Sample Output 1

9

(1,1,1),(1,1,3),(1,3,1),(1,3,3),(2,2,2),(3,1,1),(3,1,3),(3,3,1) and (3,3,3) satisfy the condition.

Sample Input 2

5 3

Sample Output 2

1

Sample Input 3

31415 9265

Sample Output 3

27

Sample Input 4

35897 932

Sample Output 4

114191

Solution in C (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <stdio.h>

int main(){
   int n,k,m,a,b,c,cn[200001],i;

   scanf("%d %d",&n,&k);
   for (i=1;i<=n;i++) cn[i]=-1;
   m=0;
   for (a=1;a<=n;a++) {
      b=k-(a%k);
      if (cn[b]>=0)
         m+=cn[b];
      else {
         i=0;c=k-b;
         if (c==0) c=k;
         for (;c<=n;c+=k)
            if ((a+c)%k==0) i++;
         m+=(cn[b]=(i*(n-b+k)/k));
      }
   }
   printf("%d\n",m);
   return 0;
}