1 #include <iostream>
2 #include <unordered_map>
3 #include <algorithm>
4 #include <vector>
5 #include <queue>
6
7 using namespace std;
8
9 int main()
10 {
11 long long int n,k;
12 while(cin >> n >> k)
13 {
14 if(n*2-1<k || k<3)
15 {
16 cout << "0" << endl;
17 }
18 else
19 {
20 if((n&0x1)==1)
21 {
22 long long mid = n/2;
23 long long dis = abs(k-(n+1));
24 cout << mid - dis/2 << endl;
25 }
26 else
27 {
28 long long mid = n/2;
29 long long dis = abs(k-(n+1));
30 cout << mid - dis%2 - dis/2 << endl;
31 }
32 }
33 }
34 return 0;
35 }
