#include <cstring>
#include <vector>
#include <stdio.h>
#include <bitset>
#include <string>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include<sstream>
#include <iomanip>
#include <vector>
#include<utility>
#include <iostream>
#define ll long long
using namespace std;
int main()
{
	ios::sync_with_stdio(0);
	cin.tie(0);
	cout.tie(0);
	ll t; cin >> t;
	while (t--) {
		ll n, k; cin >> n >> k;
		ll ctr = k / 2;
		if (n >= (k-1)) {
			if(k>2)
			cout << ctr << "\n";
			else cout << "0\n";
		}
		else if((k-1)>n) {
			ctr -= (k - n);
			ctr > 0 ? cout << ctr << "\n" : cout << "0\n";
		}
	}
}