#include <iostream>
using namespace std;
int main(){
    int n,m;cin>>n>>m;
    if(n < m || !m){
        cout << 0 << endl;
    }else{
        if(m==n+1-m){
            cout << n-1 << endl;
        }else{
            cout << n-2 << endl;
        }
    }
}