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++ (Clang 10.0.0) (judge verdict: Wrong Answer)

#include <algorithm>
#include <array>
#include <cmath>
#include <functional>
#include <iomanip>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <string>
#include <tuple>
#include <utility>
#include <vector>
#include <cassert>

#define rep(i, n) for (int i = 0; i < (int)(n); ++i)

using namespace std;

int main() {
    int N, K;
    cin >> N >> K;

    if(K%2 == 1) {
        int modK = 0;
        rep(i, N) {
            if( (i+1) % K == 0 )
                modK++;
        }
        cout << modK * modK * modK << endl;
    } else {
        int modK  = 0;
        int modK2 = 0;
        rep(i, N) {
            if((i+1) % K == 0 )
                modK++;
            if((i+1) % K == (K/2))
                modK2++;
        }
        cout << modK * modK * modK + modK2 * modK2 * modK2 << endl;
    }

    return 0;
}