#include <bits/stdc++.h>
#define rep(i, a, n) for(int i = a; i < n; i++)
#define repb(i, a, b) for(int i = a; i >= b; i--)
#define all(a) a.begin(), a.end()
#define o(a) cout << a << endl
#define int long long
#define fi first
#define se second
using namespace std;
typedef pair<int, int> P;

signed main(){
    int n, k;
    cin >> n >> k;
    int ans = 0;
    if(1 <= k && k <= n){
        ans = n - 2;
        if(n % 2 && k == (n + 1) / 2) ans = n - 1;
    }
    cout << ans << endl;
}