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<bits/stdc++.h>
using namespace std;

using ll = long long;
using ull = unsigned long long;

const double PI = acos(-1);
const int inf = 2e9;
const ll INF = 2e18;
const ll MOD = 1e9+7;

#define sz(s) (s).size()
#define pb push_back
#define fi first
#define se second
#define REP(i,n) for (int i = 0; i<n; i++)
#define ALL(a) begin(a), end(a)

int main (void) {
    int N,K;
    cin>>N>>K;
    vector<int> mm(K,0);
    for (int i=1;i<=N;i++) {
        mm[i%K]++;
    }
    ll ans = 0;
    if (mm[0]) ans += mm[0] * mm[0] * mm[0];
    if (K%2==0) ans += mm[K/2] * mm[K/2] * mm[K/2]; 
    cout<<ans<<endl;
    return 0;
}