#include <bits/stdc++.h>
#pragma GCC optimize("Ofast")
#pragma GCC optimize("unroll-loops")
#pragma GCC target("avx2")
#pragma GCC optimize("Os")
using namespace std;
int main() {
  ios::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  string str;
  long long i, j, k, x = 0, y = 0, n, m, t, sum = 0, a = 0, b, l, c, d, id;
  cin >> n;
  set<long long> s;
  for (i = 0; i < n; i++) {
    cin >> a;
    sum += a;
    s.insert(a);
  }
  x = s.size();
  if (x >= 4)
    cout << -1;
  else {
    if (x == 1)
      cout << 0;
    else if (x == 2) {
      a = *next(s.begin(), 0);
      b = *next(s.begin(), 1);
      if ((a + b) % 2 != 0)
        cout << b - a;
      else
        cout << (b - a) / 2;
    } else {
      a = *next(s.begin(), 0);
      b = *next(s.begin(), 1);
      c = *next(s.begin(), 2);
      t = c - b;
      if (a + c == b * 2)
        cout << t;
      else
        cout << -1;
    }
  }
  return 0;
}
