func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     set<int> a;     int n, b, c;     cin >> n;     for (int i = 0; i < n; i++) {       cin >> b;       a.insert(b);     }     if (a.size() == 1)       std::cout << 0 << endl;     else if (a.size() == 2) {       b = *a.begin();       c = *(--a.end());       int a1;       if ((c - b) % 2 == 1)         a1 = c - b;       else         a1 = (c - b) / 2;       cout << a1 << endl;     } else if (a.size() == 3) {       auto iter = a.begin();       int a1, a2, a3;       a1 = *iter++;       a2 = *iter++;       a3 = *iter;       if (a2 - a1 == a3 - a2)         cout << a2 - a1 << endl;       else         cout << -1 << endl;     } else       cout << -1 << endl;     return 0;   }   