#include<stdio.h>
#include<iostream>
#include<string>
#include<vector>
#include<map>
#include<list>
#include<queue>
#include<deque>
#include<algorithm>
#include<utility>
#include<memory>
#include<cmath>

#define ALL(g) (g).begin(),(g).end()
#define REP(i, x, n) for(int i = x; i < n; i++)
#define rep(i,n) REP(i,0,n)
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define pb push_back

using namespace std;
 
typedef long long ll;
typedef pair<int,int> P;
const int mod=1e9+7,INF=1<<30;
const double EPS=1e-12,PI=3.1415926535897932384626;
const int MAX_N=9;
const ll LINF=100000000000000000;

int main(){
  int N,K;
  cin >> N >> K ;
  int ans;
  if(K<=0 || K>N) ans=0;
  else if(N%2==1 && K==N/2+1) ans=N-1;
  else ans=N-2;
  
  cout << ans << endl;

  return 0;
}