#include <bits/stdc++.h>   using namespace std;   int pp(int n) {     for (int i = 2; i <= floor(sqrt(n)); i++) {       if (n % i == 0) return i;     }     return 1;   }   void solve() {     long long int n, k;     cin >> n >> k;     if (k > n)       cout << max(0ll, n - k / 2) << endl;     else       cout << max(0ll, (k - 1) / 2) << endl;   }   int main() {     int t = 1;     while (t--) solve();   }   