#include <bits/stdc++.h>
using namespace std;
int main()
{
  int t,n;
  cin >> t;
  vector<long long>a;
  bool ok;
  long long hako,left,right,mokuteki;
  for(int q=0;q<t;q++){
    a.clear();
    cin >> n;
    for(int i=0;i<n;i++){
      cin >> hako;
      a.emplace_back(hako);
    }
    ok=true;
    for(int i=0;i<n-1;i++)if(a[i]>=a[i+1])ok=false;
    if(ok){
      cout << 0 << endl;
      continue;
    }
    ok=false;
    hako=a[0]+1e9+100;
    for(int i=1;i<n;i++){
      if(hako>=a[i]+(i+1)*(1e9+100)){
	ok=true;
	break;
      }
      hako=a[i]+(i+1)*(1e9+100);
    }
    if(ok){
      cout << -1 << endl;
      continue;
    }
    left=0;
    right=1e9+100;
    while(left+1!=right){
      mokuteki=(left+right)/2;
      hako=a[0]+mokuteki;
      ok=true;
      for(int i=1;i<n;i++){
	if(hako>=a[i]+(i+1)*mokuteki){
	  ok=false;
	  break;
	}
	hako=a[i]+(i+1)*mokuteki;
      }
      if(ok)right=mokuteki;
      else left=mokuteki;
    }
    cout << right << endl;
  }
  return 0;
}