#include <algorithm>
#include <iomanip>
#include <istream>
#include <map>
#include <numeric>
#include <ostream>
#include <set>
#include <sstream>
#include <string>
#include <utility>
#include <vector>
using namespace std;
// Powered by caide (code generator, tester, and library code inliner)


class Solution {
public:
    void solve(std::istream& in, std::ostream& out) {
        int n, k;
        int ans = 0;
        in >> n >> k;
        if (k == 0 || k > n)
        {
            ans = 0;
        }
        else
        {
            ans = n == 2 * k - 1 ? n - 1 : n - 2;
        }
        out << ans;
    }
};

void solve(std::istream& in, std::ostream& out)
{
    out << std::setprecision(12);
    Solution solution;
    solution.solve(in, out);
}


#include <fstream>
#include <iostream>


int main() {
    
    ios_base::sync_with_stdio(0);
    cin.tie(0);


    istream& in = cin;


    ostream& out = cout;

    solve(in, out);
    return 0;
}