include

include<math.h>

include

using namespace std;
int main()
{
long int x,y,loop;
cin>>x;
cin>>y;

cout << fixed;
cout << setprecision(0);
if(x*2<=y )
{
    cout<<0;
}
else if(x>=y)
{

    cout<<floor(y/2);
}
else 
{
    long int out=x-ceil(y/2)  ;

    cout<<out;
}

}