#include <bits/stdc++.h>
using namespace std;
const int N = 1e6 + 10;
struct people{
    long long x, y;
}a[N];
bool pd(people p1, people p2){return p1.x < p2.x;}
long long suf[N];
int n;
long long ans = 1e18;
set<long long>se;

void upd(long long X, long long Y){
    if(se.lower_bound(X) != se.end())
         ans = min(ans, abs(max(*se.lower_bound(X), Y) - X));
    if(se.lower_bound(X) != se.begin())
        ans = min(ans, abs(max(*(--se.lower_bound(X)), Y) - X));
    
}

int main(){
    ios::sync_with_stdio(0); cin.tie(0), cout.tie(0);
    int T; cin >> T;
    while(T --){
        for (int i = 1; i <= n; ++ i) suf[i] = 0;
        cin >> n;
        for (int i = 1; i <= n; ++ i) cin >> a[i].x >> a[i].y;
        sort(a + 1, a + 1 + n, pd);
        for (int i = n; i; -- i) suf[i] = max(suf[i + 1], a[i].y);
        ans = 1e18;
        se.clear();
        a[0].x = -1;
        for (int i = 1; i <= n; ++ i){
            if(a[i].x != a[i - 1].x) upd(a[i].x, suf[i + 1]); 
            se.insert(a[i].y);
            if(a[i].x == a[i - 1].x) upd(a[i].x, suf[i + 1]); 
        }
        cout << ans << "\n";
    }
}