#include <iostream>
#include <cstdio>
#include <algorithm>
using namespace std;
typedef long long ll;
#define rep(i,n) for(int i=0;i<(n);i++)

int main(void){
	int n, k; cin >> n >> k;
	k *= 2;
	int cnt;
	if(n % k == 0){
		cnt = n / k - 1;
	}else{
		cnt = n / k;
	}
	printf("%d\n", k * cnt / 2);
	return 0;
}