#include <iostream> #include <vector> #include <string> #include <algorithm> #include <stack> #include <queue> #include <set> #include <map> using namespace std; int main(){ int n, k; cin >> n >> k; if (k > n || k <= 0) { cout << 0 << endl; } else if((n + 1)/ 2 == k && n % 2 == 1){ cout << n - 1 << endl; } else { cout << n - 2 << endl; } }