#include<bits/stdc++.h>
using namespace std;
#define int long long
using pii=pair<int,int>;
const int N=1e6+5;
const int INF=1e9+117;
int n,t[N],c[N];
vector<pair<int,pii>>arr;
bool itc(int l1,int r1,int l2,int r2){return max(l1,l2)<=min(r1,r2);}
bool DtL(int d,int l,int r,int len)
{
   // cout<<d<<','<<l<<','<<r<<','<<len<<','<<itc(d-len,d+len,l,r)<<"***\n";
    return itc(d-len,d+len,l,r);
}
bool check(int v){
    //int rx=1,ry=INF,lx=1,ly=INF;
    int lst=0;\
    int p=arr[0].second.first,l=1,r=INF;//1:r_fx
    //cout<<v<<"---------------------\n";
    for(auto&k:arr)
    {
        int t=k.first,pl=k.second.first,pr=k.second.second;

        int len=v*(t-lst);lst=t;
        //cout<<t<<"*"<<len<<"|"<<pl<<"&"<<pr<<'\n';
        if(pr!=-1)
        {
            if(DtL(pl,p,p,len)&&DtL(pr,l,r,len))l=r=pl,p=pr;
            else if(DtL(pr,p,p,len)&&DtL(pl,l,r,len))l=r=pl,p=pr;
            else return 0;
        }
        else
        {
            bool dok=DtL(pl,p,p,len),lok=DtL(pl,l,r,len);
            //cout<<dok<<"%%"<<lok<<'\n';
            int p2=p,l2,r2;
            if(dok&&lok)
            {
                r2=max(p,r)+len,l2=min(p,l)-len;
            }
            else if(dok)
            {
                r2=r+len,l2=l-len;
            }
            else if(lok)
            {
                r2=p+len,l2=p-len;
            }
            else return 0;
            l=l2,r=r2,p=p2;
        }
        //cout<<p<<','<<l<<','<<r<<'\n';
       // cout<<"^^^^^^^\n";
    }
    return 1;
}
void solve()
{
    cin>>n;arr.clear();

    for(int i=1;i<=n;i++){
        cin>>t[i]>>c[i];
    }
    pii p={-1,-1};int lst=t[1];
    for(int i=1;i<=n;i++)
    {
        if(lst==t[i])
        {
            if(p.second!=-1){cout<<"-1\n";return;}
            if(p.first==-1)p.first=c[i];else p.second=c[i];
        }
        else
        {
            arr.push_back({lst,p});
            lst=t[i];p={c[i],-1};
        }
        if(i==n)arr.push_back({lst,p});
    }
    for(auto&k:arr)
    {
        auto&[a,b]=k.second;
        if(b!=-1&&a>b)swap(a,b);
        //cout<<k.first<<','<<a<<','<<b<<'\n';
    }
    // cout<<"**\n";
    int l=-1,r=INF,mid;
    while(r-l>1){
        mid=(l+r)/2;
        if(check(mid)){
            r=mid;
        }else{
            l=mid;
        }
    }
    if(r>INF-10)r=-1;

    cout<<r<<'\n';
}
signed main() {
    ios::sync_with_stdio(0);
    cin.tie(0);cout.tie(0);
    int t=1;cin>>t;
    while(t--)solve();

    return 0;
}