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++14 (GCC 5.4.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <utility>
#include <algorithm>
#include <cmath>
#include <cstdint>
using namespace std;

int main(){
    int N, K;

    cin >> N >> K;

    int multiple_num = (2 * N) / K;

    int ans = 0;
    for(int i = 1; i <= multiple_num; i++) {
        int target_i = i * K;
        for(int j = 1; j <= min(target_i, N); j++) {
            int a = j;
            int b = target_i - j;
            if(b < a) {
                for(int k = a / K; k <= multiple_num; k++) {
                    int target_k = k * K;
                    int c = target_k - a;
                    if(0 < c && c <= N && (c + b) % K == 0) {
                        ans++;
                    }
                }
            }
            else {
                for(int k = b / K; k <= multiple_num; k++) {
                    int target_k = k * K;
                    int c = target_k - b;
                    if(0 < c && c <= N && (c + a) % K == 0) {
                        ans++;
                    }
                }
            }
        }
    }

    cout << ans << endl;

    return 0;
}