/*** WORK : 省赛\桂林ccpc\G.cpp ***/
/*** TIME : 2024/12/22 16:41 ***/
#include <bits/stdc++.h>
using namespace std;
#define int long long

#define all(v) v.begin(), v.end()
#define eb emplace_back
#define se second
#define fi first
#define endl '\n'

using vi = vector<int>;using vvi = vector<vi>;using vvvi = vector<vvi>;
using pii = pair<int,int>;using vpii = vector<pii>;using vvpii = vector<vpii>;
using mii = map<int,int>;using mpi = map<pii,int>;using si = set<int>;
istream& operator>>(istream& in,vi& a){for(int &x:a)in>>x;return in;}
ostream& operator<<(ostream& out,const vi& a){for(int x:a)out<<x<<" ";return out;}
const int INF = LLONG_MAX/4;const int MOD = (int)1e9 + 7;
const int MAXN = 2e5 + 5;

int cnt = 0;
void solve()
{
    int n,t;cin >> n;
    vi a(n);
    cin >> a;
    sort(all(a));
    a.resize(unique(all(a))-a.begin());
    n = a.size();
    cout << "Case " << ++cnt << ": ";

    if(n==1){
        if(a[0]==1)cout << 1 << endl;
        else cout << 0 << endl;
        return;
    }
    int g = 0;
    for(int i=1;i<n;i++){
        g = __gcd(g,a[i]-a[i-1]);
    }
    // cout  << g << endl;
    if(g==1){
        cout << -1 << endl;
    }else{
        int ans = INF;
        for(int i=2;i*i<=g;i++){
            if(g%i)continue;
            ans = min(ans,(i - a[0]%i)%i);
            ans = min(ans,(g/i -  a[0]%(g/i))%(g/i));
        }
        ans = min(ans,(g - a[0]%g)%g);
        cout << ans << endl;
    }
    // cout << __gcd(a[0],a[1]);

}

int32_t main()
{
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    cout << fixed << setprecision(12);
    int T = 1;
    cin >> T;
    while (T--)
        solve();
}