#include <iostream>
#include <vector>
#include <algorithm>
#include <math.h>
#include <map>

#define ALL(x) (x).begin(),(x).end()
#define RALL(x) (x).rbegin(),(x).rend()
#define REP(i,a,n) for(int i = (a);i < (n);i++)
#define PI 3.14159265359
#define MOD 1000000007

using namespace std;

int main(){
    int n,k;
    cin >> n >> k;
    if(k < 1 || 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;
}