#include <bits/stdc++.h>   using namespace std;   int main() {     int a[101], dif[100], i, unique[101], c = 0;     int min = 0, n, dif1, dif2;     cin >> n;     for (i = 0; i < n; i++) {       cin >> a[i];     }     sort(a, a + n);     unique[c] = a[0];     c++;     for (i = 1; i < n; i++) {       if (a[i] != a[i - 1]) {         unique[c] = a[i];         c++;       }     }     if (c >= 4) {       cout << -1;     } else if (c == 1) {       cout << 0 << endl;     } else if (c == 2) {       if ((unique[1] - unique[0]) % 2) {         cout << unique[1] - unique[0];       } else {         cout << (unique[1] - unique[0]) / 2;       }     } else {       if (unique[2] - unique[1] == unique[1] - unique[0]) {         cout << unique[2] - unique[1];       } else {         cout << -1 << endl;       }     }   }   