// It's a wonderful life.
#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define DEBUG 1
const ll N = 2000000;
const ll MOD = 998244353;
const ll MAX = 1e18;

ll n,a[N];
bool check(ll x){
    if (x <= 1){
        return 0;
    }
    for (ll i = 2;i*i<=x;i++){
        if (x % i == 0){
            return 0;
        }
    }
    return 1;
}

void solve(int idx){
    cin >> n;
    for (int i = 1;i<n+1;i++){
        cin >> a[i];
    }
    sort(a + 1,a + 1 + n);
    ll x = 0;
    for (int i = 2;i<n+1;i++){
        x = __gcd(x,a[i] - a[i-1]);
    }

    if (x == 0){
        if (a[1] == 1){
            cout << "Case " << idx << ": " << 1 << endl;
        }else{
            cout << "Case " << idx << ": " << 0 << endl;
        }
        return;
    }

    vector<ll> q;

    for (ll i = 2;i*i<=x;i++){
        if (x % i == 0) q.push_back(i);
        while (x % i == 0) x /= i;
    }
    if (x > 1){
        q.push_back(x);
    }

    ll ans = MAX;
    for (auto v : q){
        ll res = 0;
        res = v - a[1] % v;
        res %= v;
        ans = min(ans,res);
    }

    if (ans == MAX){
        ans = -1;
    }

    cout << "Case " << idx << ": " << ans << endl;
    // cout << ans << endl;
    return;
}

signed main(){
    // freopen("input.txt","r",stdin);
    ios::sync_with_stdio(0),cin.tie(0),cout.tie(0);
    int _ = 1;
    cin >> _;
    for (int i = 1;i<_+1;i++){
        solve(i);
    }
    return 0;
}