#include <iostream>
#include <algorithm>
#include <functional>
#include <string>
#include <climits>
#include <vector>
#include <numeric>
#include <complex>
#include <map>
#include <bitset>
#include <stack>
#include <queue>
#include <set>
using namespace std;

//#define __int64 long long
#define REP(i,a,b) for(int i=a;i<b;i++)
#define rep(i,n) REP(i,0,n)
const int Vec = 4;
const int Vecy[Vec] = {0,-1,0,1};
const int Vecx[Vec] = {1,0,-1,0};
const int Modd = 1000000007;

int main(){
	int n,k;
	cin >> n >> k;
	
	if(k == 0 || k > n){
		cout << 0 << endl;
	}else if(n % 2 == 1 && n / 2 + 1 == k){
		cout << n - 1 << endl;
	}else{
		cout << n - 2 << endl;
	}
	
    return 0;
}