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>
#define rep(i,n) for(int i=0;i<n;i++)
#define all(x) (x).begin(),(x).end()
using namespace std;
const int INF=1e9,MOD=1e9+7;

int main(){
	int n,k; cin>>n>>k;
	long long cnt1=0,cnt2=0;
	if(k%2==0){
		for(int i=1;i<=n;i++){
			if(i%k==k/2) cnt1++;
			if(i%k==0) cnt2++;
		}
		long long a=pow(cnt1,3);
		long long b=pow(cnt2,3);
		cout<<a+b<<endl;
	}else{
		for(int i=1;i<=n;i++){
			if(i%k==0) cnt2++;
		}
		long long b=pow(cnt2,3);
		cout<<b<<endl;
	}
	return 0;
}