func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     set<int> s;     int n;     cin >> n;     for (int i = 0; i < n; i++) {       int x;       cin >> x;       s.insert(x);     }     set<int>::iterator it, it1, it2;     it = s.begin();     it1 = it;     it1++;     it2 = s.end();     it2--;     if (s.size() == 1)       cout << 0 << endl;     else if (s.size() == 2) {       int e = *it1 - (*it);       if (e % 2 == 0) e /= 2;       cout << e;     } else if (s.size() == 3) {       int a = (*it);       int b = (*it1);       int c = (*it2);       if (b - a == c - b)         cout << b - a << endl;       else         cout << -1 << endl;     } else       cout << -1 << endl;   }   