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

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

	return 0;
}