TOI10: Fortress

#include <iostream>
using namespace std;

int main(){

    for(int i = 1; i <= 20; ++i){
        int sum, tr;
        scanf("%d", &sum);
        scanf("%d", &tr);
        if(sum % 2 == 0){
            int x = (sum / 2) - tr;
            int r = tr - x;
            if(x >= 0 && r >= 0){
                cout << r + 1 << "\n";
                continue;
            }
        }
        cout << 0 << "\n";
    }

    return 0;
}