#include <bits/stdc++.h>
#include "bits/stdc++.h"
#include <iostream>
#include <iomanip>
#include <vector>
#include <algorithm>
#include <string>
#include <sstream>
#include <cmath>
#include <stack>
#include <queue>
#include <cctype>
#include <stdio.h>
#include <map>
#include <unordered_map>
#include <string.h>
#include <utility>

typedef long long ll;
const int INF = 1e8;

#define rep(i,n) for(ll i=0;i<(ll)(n);i++)
using namespace std;

typedef pair<int, int> P;


int main() {

	int n,k;
	cin>>n>>k;

	if(k ==0 || k>n){
		cout<<0<<endl;
		return 0;
	}

	if(n %2 ==0){
		cout<<n-2<<endl;
		return 0;
	}

	if(n/2+1 == k){
		cout<<n-1<<endl;
	}else {
		cout<<n-2<<endl;
	}

return 0;

}