#include <bits/stdc++.h>
using namespace std;

int main(){
    int N, K;

    cin >> N >> K;

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

    return 0;
}