#include <iostream>
#include <vector>
using namespace std;
 
int main(){
    int N, K;
    cin >> N >> K;
    if(K < 1 || K > N){
        cout << 0 << endl;
    }else if(K * 2 - 1 == N){
        cout << N - 1 << endl;
    }else{
        cout << N - 2 << endl;
    }
}