#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); i++)
#define shosu(n) setprecision(n)
using namespace std;
int main() {
  int N, M;
  cin >> N >> M;
  if (M == 0 || N < M)
    cout << 0 << endl;
  else if (N % 2 == 0)
    cout << N - 2 << endl;
  else if (N % 2 == 1) {
    if (M == N / 2 + 1)
      cout << N - 1 << endl;
    else
      cout << N - 2 << endl;
  }
  return 0;
}