#include<bits/stdc++.h>
using namespace std;

typedef long long int           ll;
typedef unsigned long long int  ull;
typedef vector<int>             vi;
typedef vector<vi>              vvi;
typedef pair<ll,ll>             pll;
typedef vector< pll >           vpll;
typedef set<int>                sti;

#define pb                      push_back
#define pob                     pop_back
#define mp                      make_pair
#define ff                      first
#define ss                      second
#define loop1(i,n)              for(int i=1; i<=int(n); i++)
#define loop(i,y)               for(int i=0; i<int(y); i++)
#define rloop(i,y)              for(int i=int(y); i>0; i--)
#define rloop1(i,y)             for(int i=int(y)-1; i>=0; i--)
#define read()                  freopen("input.txt", "r", stdin)
#define write()                 freopen("output.txt", "w", stdout)
#define cspf(i)                 printf("Case %d: ", i)
/// Constants
#define eps                     1e-9
#define PI                      acos(-1.0)  // 3.1415926535897932

ll a[100009];
int main()
{
    ll n,i,j,tgc,gc=0;
    cin>>n;
    loop(i,n){
        cin>>a[i];
    }
    for(i=0,j=n-1;i<=j;i++,j--){
        if(a[i]!=a[j]){
            tgc=abs(a[i]-a[j]);
            if(gc==0)
                gc=tgc;
            else
                gc=__gcd(gc,tgc);
        }
    }
    if(gc)
        cout<<gc<<endl;
    else
        cout<<"-1"<<endl;
    return 0;
}