#include<string>
#include<deque>
#include<queue>
#include<vector>
#include<algorithm>
#include<iostream>
#include<set>
#include<cmath>
#include<tuple>
//#include<map>
//65 1000000000000000000 1000000000000000000
using namespace std;
typedef long long int llint;
typedef int beet;
#define ween tuple
#define mp make_pair
#define mt make_tuple
#define pub push_back
#define puf push_front
#define pob pop_back
#define pof pop_front
#define izryt bool
#define fir first
#define sec second
#define res resize
#define ins insert
#define era erase
const llint mod=1000000007;
const llint big=1e9+1e8;
const llint red=0xE869120;
const llint pro=1002001;

int main(void){
	int i,j,ans=0,q,w;
	cin>>q>>w;
	if(w<=0||w>q){cout<<0<<endl;return 0;}
	if(q+1==w*2){cout<<q-1<<endl;return 0;}
	cout<<q-2<<endl;
	return 0;
}