#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <math.h>
using namespace std;

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