codeforces-1023 B Pair of Toys

 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 }