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: Accepted)

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

#define rep(i,n) for(int (i)=0;(i)<(n);(i)++)
#define reps(i,s,n) for(int (i)=(s);(i)<(n);(i)++)
#define repr(i,n) for(int (i)=(n)-1;(i)>=0;(i)--)
#define all(v) (v).begin(),(v).end()
#define chmax(a, b) a = (((a)<(b)) ? (b) : (a))
#define chmin(a, b) a = (((a)>(b)) ? (b) : (a))
#define pb push_back
#define mod 1000000007
#define inf 1000000000
#define llinf 1000000000000000

typedef long long ll;
typedef vector<int> vint;
typedef vector<ll> vll;
typedef pair<int,int> pint;
typedef pair<ll,ll> pll;
typedef vector<pint> vpint;

int func(int a,int b){
    //if(a<b)return func(b,a);
    if(b==0)return a;
    return func(b,a%b);
}

int main(){
    int n,k;
    cin>>n>>k;
    if(k%2==1){
        cout<<(ll)(n/k)*(ll)(n/k)*(ll)(n/k)<<endl;
    }
    else{
        cout<<(ll)((n+(k/2))/k)*(ll)((n+(k/2))/k)*(ll)((n+(k/2))/k)+(ll)(n/k)*(ll)(n/k)*(ll)(n/k)<<endl;
    }
}