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;
#define INFTY 1<<20
typedef long long int ll;
const int MOD=1000000007;
using Graph = vector<vector<int>>;
using ipair = pair<ll,ll>;
bool operator< (const ipair a, const ipair b){return a.first < b.first;};
#define rep(i,n) for (int i = 0; i < (n); ++i)
ll gcd(ll a, ll b) { return b?gcd(b,a%b):a;}
ll lcm(ll a, ll b) { return a/gcd(a,b)*b;}

int main(){
  int n,k;cin>>n>>k;
  int l,m;
  l = n/k;
  if(k%2==0)m=l;
  else m=0;
  if(n-l>k/2&&k%2==0)m++;
  ll ans;
  ans = l*l*l;
  ans += m*m*m;
  cout<<ans;
  return 0;
}