#include <bits/stdc++.h>
using namespace std;

//--------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define RFOR(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define REP(i,n) FOR(i,0,n)
#define RREP(i,n) RFOR(i,0,n)
#define LL long long
#define INF INT_MAX/3

//--------------------------------------------
const double EPS = 1e-14;
const double PI  = acos(-1.0);

//--------------------------------------------

int main(){
  int n,k;
  int ans;
  scanf("%d %d",&n,&k);

  if (k == 0 || n < k) {
    ans = 0;
  } else if (n % 2 == 1) {
    if (n / 2 == k - 1) ans = n - 1;
    else ans = n - 2;
  } else ans = n - 2; 

  printf("%d\n",ans);

}