1154B.cpp

#include <bits/stdc++.h>
#include <ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/assoc_container.hpp>

using namespace std;
using namespace __gnu_pbds;
typedef long long ll;

struct int_hash {
    static uint64_t splitmix64(uint64_t x) {
        x += 0x9e3779b97f4a7c15;
        x = (x ^ (x >> 30)) * 0xbf58476d1ce4e5b9;
        x = (x ^ (x >> 27)) * 0x94d049bb133111eb;
        return x ^ (x >> 31);
    }

    size_t operator()(uint64_t x) const {
        static const uint64_t FIXED_RANDOM = chrono::steady_clock::now().time_since_epoch().count();
        return splitmix64(x + FIXED_RANDOM);
    }
};

struct pair_hash {
    size_t operator()(pair<int, int> x) const {
        return int_hash{}(x.first) ^ (int_hash{}(x.second) << 16);
    }
};

mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());

const int MOD = 1000000007;
const ll INF = numeric_limits<ll>::max();
const int inf = 1e7;
const int MX = 100001; //check the limits, dummy

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);

    int n; cin >> n;
    set<int> s;
    for(int i = 0; i < n; i++) {
        int in; cin >> in; s.insert(in);
    }

    vector<int> v(s.begin(), s.end());
    if(v.size() == 1) cout << 0 << endl;
    else if(v.size() == 2) { 
        if((v[0] & 1) == (v[1] & 1)) cout << v[1] - (v[0] + v[1]) / 2 << endl; 
        else cout << v[1] - v[0] << endl;
    }
    else if(v.size() == 3 && v[1] - v[0] == v[2] - v[1]) cout << v[1] - v[0] << endl; 
    else cout << -1 << endl;
}