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 rep(i, n)  for(long long i=0;i<(long long)(n);i++)
#define REP(i,k,n) for(long long i=k;i<(long long)(n);i++)
#define all(a) a.begin(),a.end()
#define eb emplace_back
#define pb push_back
#define lb(v,k) (lower_bound(all(v),k)-v.begin())
#define chmin(x,y) if(x>y)x=y
#define chmax(x,y) if(x<y)x=y
typedef long long ll;
typedef pair<ll,ll> P;
typedef tuple<ll,ll,ll> PP;
typedef priority_queue<ll> PQ;
typedef priority_queue<ll,vector<ll>,greater<ll>> SPQ;
using vi=vector<ll>;
using vvi=vector<vector<ll>>;
using vc=vector<char>;
using vvc=vector<vector<char>>;
const ll inf=1001001001001001;
const int Inf=1001001001;
const int mod=1000000007;
int main(){
    int n,k,ans=0;cin>>n>>k;
    ans=n/k;
    ans=ans*ans*ans;
    if(k%2==0){
        ll res=(n/(k/2));
        res=(res+1)/2;
        ans+=res*res*res;
    }
    cout<<ans<<endl;
}