AllAreTheSame.cc

#ifndef LOCAL
#pragma GCC optimize(2)
#endif
#include <bits/stdc++.h>
using namespace std;
typedef long long int ll;
typedef vector<ll> vi;
typedef pair<ll,ll> ii;

#define pb push_back
#define fst first
#define snd second
#define ALL(cont) cont.begin(), cont.end()
#define mset(a,b) memset(a,b,sizeof(a));
#define fore(i, a, b) for (int i = a, almo5t = b; i < almo5t; ++i)
#define SZ(x) ((int)x.size())
#define FIN ios::sync_with_stdio(false);cin.tie(0);cout.tie(0)

int main()
{
    FIN;
#ifdef LOCAL
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
#else
#define endl '\n'
#endif
    int t;cin>>t;
    while(t--){
        int n;cin>>n;
        vi a(n);
        fore(i,0,n)cin>>a[i];
        int m = *min_element(ALL(a));
        vi a2;
        fore(i,0,n)if(a[i]!=m)a2.pb(a[i]-m);
        ll ans = 0;
        if(SZ(a2)){
            ans=a2[0];
            fore(i,0,SZ(a2))ans = __gcd(ans,a2[i]);
        }
        cout<<(ans?ans:-1)<<endl;
    }
    
    return 0;
}