#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 (int i=0;i<(n);i++)
#define RREP(i,n) for (int i=(n)-1;i>=0;i--)
typedef long long LL;
LL N,K;
int main(){
    cin>>N>>K;
    if(K==0){
       cout<<0<<endl;
    }else if(K>N){
       cout<<0<<endl;
    }else{
       if(2*K-1==N){
          cout<<N-1<<endl;
       }else{
          cout<<N-2<<endl;
       }
    }
    return(0);
}