#include <bits/stdc++.h>
#define REP(i, a, n) for(ll i = ((ll) a); i < ((ll) n); i++)
using namespace std;
typedef long long ll;

int main(void) {
    ll N,K;
    ll ans;
    cin >> N >> K;
    if (K < 1 || K > N) ans = 0;
    else if (N % 2 == 1 && N == K * 2 - 1) ans = N - 1;
    else ans = N -2;
    cout << ans << endl;
    return 0;
}