//#define LOCAL

#include <fstream>
#include <iostream>
#include <cmath>
#include <algorithm>
#include <vector>
#include <map>
#include <queue>
#include <cstring>
#include <climits>

//#define int long long
//typedef long long ll;
#define rep(i,n) for(int i=0; i<(n); i++)

#define MOD 1000000007

using namespace std;



signed main()
{
#ifdef LOCAL
    ifstream in("input.txt");
    cin.rdbuf(in.rdbuf());
#endif

    int N, K;

    cin >> N >> K;

    if (K == 0 || K > N) {
        cout << 0 << endl;
        return 0;
    }

    if (N % 2 == 0) {
        cout << N - 2 << endl;
    } else {
        if (K == N / 2 + 1) {
            cout << N - 1 << endl;
        } else {
            cout << N - 2 << endl;
        }
    }
}