#include <stdio.h>
int N,D;
int main(){
  int ans;
  scanf("%d %d",&N,&D);
  if(N%2==0){
    if(D==1)ans=N-1;
    else if(D==N)ans=0;
    else ans=N/2;
  }
  else{
    if(D==N)ans=0;
    else ans=N-1;
  }
  printf("%d\n",ans);
  return 0;
}