#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int Mod=1e9+7;
const ll INF = 10000000000000;
const int N = 1e6+7;
 
    ll s(ll a,ll b,ll c,ll d,ll n){
        ll w=b-a+1;
        ll h=d-c+1;
        if(w*h<n) return min((w+1)*h,w*(h+1));
        return w*h;
    }
 
    void solve() {
        int a,b;
        cin >> a >> b;
        if(a>=b) cout << a << '\n';
        else if(2*a<=b) cout << 0 << '\n';
        else cout << 2*a-b << '\n';
      }       
 
int main(){ 
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
 
    int t;
    cin >> t;
    while (t--) solve();
 
    return 0;
}
 

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5
10 5
7 9
5 100
1 1
1 2