#include <iostream>
using namespace std;

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