#include<iostream>
#include<queue>
#include<map>
#include<set>
#include<vector>
#include<algorithm>
#include<deque>
#include<cctype>
#include<string.h>
#include<math.h>
#include<time.h>
#include<random>
#include<functional>
#include<stack>
#include<unordered_map>
#include<string>
#define ll                            long long 
#define lowbit(x) (x & -x)
#define endl "\n"//                           交互题记得删除
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// using namespace __gnu_pbds;
using namespace std;
mt19937 rnd(time(0));
const ll mod = 998244353;
//const ll p=rnd()%mod;
#define F first
#define S second
// tree<ll,null_type,less<ll>,rb_tree_tag,tree_order_statistics_node_update>;find_by_order(k)，找第k位（从小到大）的数字，order_of_key(x)，x的排名
ll ksm(ll x, ll y)
{
    ll ans = 1;
    x %= mod;
    while (y)
    {
        if (y & 1)
        {
            ans = ans * x % mod;
        }
        x = x * x % mod;
        y >>= 1;
    }
    return ans;
}
ll gcd(ll x, ll y)
{
    if (y == 0)
        return x;
    else
        return gcd(y, x % y);
}
inline ll read()
{
    register ll x = 0, f = 1;
    char c = getchar();
    while (c < '0' || c>'9')
    {
        if (c == '-') f = -1;
        c = getchar();
    }
    while (c >= '0' && c <= '9')
    {
        x = (x << 3) + (x << 1) + (c ^ 48); //等价于x*10+c-48,使用位运算加速
        c = getchar();
    }
    return x * f;
}
void fio()
{
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);
}
// struct s
// {
//     ll l,r;
//     friend bool operator<(cosnt s &a, const s& b)
//     {
//         return 
//     };
// };
int main()
{
    fio();
    ll t;
    cin >> t;
    ll cs=0;
    while (t--)
    {
        cs++;   
        ll n;
        cin>>n;
        vector<ll>a(n+5);set<ll>k;
        ll d;
        for(ll i=1;i<=n;i++)
        {
            cin>>a[i];
            k.insert(a[i]);
        }
        cout<<"Case"<<" "<<cs<<": ";
        if(k.size()==1)
        {
            ll ans=0;
            if((*k.begin())==1)ans++;
            cout<<ans<<endl;
        }
        else 
        {
            ll f=-1;
            ll z=0;
            for(auto j:k)
            {
                if(f==-1)f=j;
                else 
                {
                    z=gcd(z,j-f);
                    f=j;
                }
            }
            if(z==1)cout<<-1<<endl;
            else 
            {
                d=*k.begin();
                ll ans=1e18;
                if(d%z==0)ans=0;
                else ans=(d/z+1)*z-d;
                for(ll i=2;i*i<=z;i++)
                {
                    if(z%i==0)
                    {
                        ll x=z/i;
                        if(d%i==0||d%x==0)ans=0;
                        else 
                        {
                            ans=min({ans,(d/i+1)*i-d,(d/x+1)*x-d});
                        }
                    }
                }
                cout<<ans<<endl;
            }
        }
    }
}