#include<iostream>
#include<string>
#include<algorithm>
#include<cctype>
#include<set>
#include<bitset>
#include<math.h>
#include<map>
#include<queue>
#include<iomanip>
using namespace std;
#ifdef _WIN32
#include <Windows.h>
#endif

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