#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
using ll = long long;
using ld = long double;

ll N, K;

int main() {
  cin >> N >> K;
  if (K == 0 || K > N || K == 1 && N == 1) cout << 0 << endl;
  else if (N % 2 == 1) {
    if (K == N / 2 + 1) cout << N - 1 << endl;
    else cout << N - 2 << endl;
  } else {
    cout << N - 2 << endl;
  }
  return 0;
}