/*
Look at the star
Look at the shine for U
*/
#include<bits/stdc++.h>
#define ll long long
#define PII pair<int,int>
#define sl(x) scanf("%lld",&x)
using namespace std;
const int N = 1e6+5;
const int mod = 1e9+7;
const int INF = 0x3f3f3f3f;
const double PI = acos(-1);
ll inv(ll b){if(b==1)return 1; return (mod-mod/b)*inv(mod%b)%mod;}
ll fpow(ll n,ll k){ll r=1;for(;k;k>>=1){if(k&1)r=r*n%mod;n=n*n%mod;}return r;}
ll s[N],fac[N],cnt;
int main()
{
    ll n,i,j,k,sum,t,cas = 1;
    sl(t);
    while(t--)
    {
        sl(n);
        sum = 0;
        for(i = 0;i < n;i++){sl(s[i]),sum += s[i];}
        printf("Case %lld: ",cas++);
        if(sum == n)
        {
            puts("1");
            continue;
        }
        sort(s,s+n);
        ll flag = 0;
        for(i = 0;i < n;i++)
        {
            if(s[i]%s[0]) flag = 1;
        }
        if(!flag && s[0] != 1)
        {
            puts("0");
            continue;
        }
        ll Gcd = 0;
        for(i = 1;i < n;i++)
        {
            ll dis = s[i]-s[i-1];
            if(dis == 0) continue;
            Gcd = __gcd(Gcd,dis);
        }
        if(Gcd == 1)
        {
            puts("-1");
            continue;
        }
        flag = 0;
        for(i = 0;i < n;i++)
        {
            if(s[i]%Gcd) flag = 1;
        }
 
        if(!flag)
        {
            puts("0");
            continue;
        }
        cnt = 0;
        for(i = 2;i*i <= Gcd;i++)
        {
            if(Gcd%i == 0)
            {
                fac[cnt++] = i;
                fac[cnt++] = Gcd/i;
            }
        }
        fac[cnt++] = Gcd;
        sort(fac,fac+cnt);
        ll minn = 1e18;
        ll now = 0;
        for(i = 0;i < cnt;i++)
        {
            ll temp = 0;
            for(j = 0;j < n;j++)
            {
                if(s[j]%fac[i]) temp = 1; 
            }
            if(!temp)
            {
                now = 1;
                break;
            }
        }
        if(now)
        {
            puts("0");
            continue;
        }
        for(i = 0;i < cnt;i++)
        {
            minn = min(minn,fac[i]-s[0]%fac[i]);
        }
        printf("%lld\n",minn);
    }
}