codeforces-1023 B Pair of Toys

#include <iostream>
#include <unordered_map>
#include <algorithm>
#include <vector>
#include <queue>

using namespace std;

int main()
{
    long long int n,k;
    while(cin >> n >> k)
    {
        if(n*2-1<k || k<3)
        {
            cout << "0" << endl;
        }
        else
        {
            if((n&0x1)==1)
            {
                long long mid = n/2;
                long long dis = abs(k-(n+1));
                cout << mid - dis/2 << endl;
            }
            else
            {
                long long mid = n/2;
                long long dis = abs(k-(n+1));
                cout << mid - dis%2 - dis/2 << endl;
            }
        }
    }
    return 0;
}