#include<iostream>
#include<algorithm>
using namespace std;
const int N=1e6+10;
int t,a[N],b[N];
int main(){
//freopen("interval.in","r",stdin);
//freopen("interval.out","w",stdout);
cin>>t;
while(t--){
int n;
cin>>n;
for(int i=1;i<=n;i++) cin>>a[i];
for(int i=1;i<n;i++) b[i]=a[i+1]-a[i];
int v=b[1];
for(int i=2;i<n;i++){
if(b[i]==0){
v=0;
break;
}
v=__gcd(v,b[i]);
}
int flog=1;
for(int i=2;i<=n;i++){
if(a[1]!=a[i]){
flog=0;
break;
}
}
if(v==0 && !flog){
cout<<"-1"<<endl;
continue;
}
if(flog){
cout<<0<<endl;
continue;
}
int ans=0;
for(int i=1;i<n;i++){
ans+=(b[i]-v)/v;
}
cout<<ans<<endl;
}
return 0;
}