#include<bits/stdc++.h> using namespace std; using ll = long long; void solve() { int n; cin>>n; vector<int>v(n+1); for(int i=1; i<=n; i++) { cin>>v[i]; } int ans=0, diff; for(int i=1; i<=n; ++i) { diff=abs(v[i]-i); ans=__gcd(ans,diff); } cout<<ans<<endl; } int main() { ios_base::sync_with_stdio(false); cin.tie(nullptr); int TC = 1; cin >> TC; cin.ignore(); while (TC--) solve(); }