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 9.2.1) (judge verdict: Wrong Answer)

#include <iostream>
#include <vector>
#include <string> 
#include <algorithm>
#include <set>
using namespace std;
int main(){
    int N, K;
    cin >> N >> K;
    vector<int> cnt(K);
    for(int i = 1; i <= N; i++){
        cnt[i % K]++;
    }
    int ans = 0;
    for(int i = 1; i <= N; i++){
        if((K - i % K) % K * 2 % K != 0) continue;
        ans += cnt[(K - i % K) % K] * cnt[(K - i % K) % K];
    }
    cout << ans << endl;
}