#include <bits/stdc++.h>   using namespace std;   int n, num, _ = 0;   int a[1005];   int main() {     cin >> n;     map<int, int> mp;     for (int i = 0; i < 100000; i++) _++;     int m = 0;     for (int i = 0; i < 100000; i++) _++;     for (int i = 0; i < n; i++) {       int num;       cin >> num;       if (mp[num] == 0) {         mp[num]++;         a[m++] = num;       }     }     for (int i = 0; i < 100000; i++) _++;     sort(a, a + m);     for (int i = 0; i < 100000; i++) _++;     if (m == 1)       cout << 0 << endl;     else if (m == 2) {       if ((a[1] - a[0]) % 2 == 1)         cout << a[1] - a[0] << endl;       else         cout << (a[1] - a[0]) / 2 << endl;     } else if (m == 3) {       if (a[2] - a[1] == a[1] - a[0])         cout << a[1] - a[0] << endl;       else         cout <<  -1  << endl;     } else       cout <<  -1  << endl;     for (int i = 0; i < 100000; i++) _++;     return 0;   }   