#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using P = pair<int,int>;
#define rep(i,n) for(int i=0;i<n;i++)

const ll mod = ll(1e9) + 7;
const int INF = 1<<29;

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