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: Time Limit Exceeded)

#include <stdio.h>

int main(int argc, char *argv[])
{
    int N, K;
    int a, b, c;
    int ans = 0;
    
    scanf("%d%d", &N, &K);
    for (a = 1; a <= N; a++) {
        for (b = K - a; b <= N; b += K) {
            if (b <= 0)
                continue;
            for (c = K - b; c <= N; c += K) {
                if ((c >= 1) && ((a + c) % K == 0))
                    ans++;
            }
        }
    }
    printf("%d\n", ans);
    
    return 0;
}