#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<vector>
#include<string>
#include<sstream>
#include<cmath>
#include<numeric>
#include<map>
#include<stack>
#include<queue>
using namespace std;


int main(void) {

  int n, k;
  cin >> n >> k;
  if( k <= 0 || k > n ){
    cout << 0 << endl;
  }else{
    if( k == n - k + 1 ) cout << n - 1 << endl;
    else cout << n - 2 << endl;
  }

  return 0;
}






// EOF