#include <bits/stdc++.h>
using namespace std;
 
typedef long long ll;
#define REP(i,n) for(int i=0,_n=(int)(n);i<_n;++i)
template<class T>bool chmin(T&a,const T&b){return a>b?(a=b,true):false;}
template<class T>bool chmax(T&a,const T&b){return a<b?(a=b,true):false;}

int nextInt() {
  int x; scanf("%d", &x); return x;
}

int main2() {
  ll N = nextInt();
  ll K = nextInt();

  ll ans = 0;
  if (K == 0 || K > N)
    ans = 0;
  else if (K == N + 1 - K) {
    ans = N - 1;
  } else {
    ans = N - 2;
  }
  cout << ans << endl;

  return 0;
}

int main() {
  for (;!cin.eof();cin>>ws) main2();
  return 0;
}