#include <bits/stdc++.h>   using namespace std;   void print(vector<long long> v) {     long long n = v.size();     for (long long i = 0; i < n; i++) {       cout << v[i] <<    ;     }   }   long long power(long long a, long long b) {     long long res = 1;     while (b != 0) {       if (b & 1) {         res *= a;       }       a = (a * a);       b = (b / 2);     }     return res;   }   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     cout.tie(NULL);     ;     long long n;     cin >> n;     vector<long long> v(n);     for (long long i = 0; i < n; i++) {       cin >> v[i];     }     if (n <= 3) {       cout << 1;       return 0;     }     vector<long long> v1 = v;     sort(v1.begin(), v1.end());     vector<long long> diff;     map<long long, vector<long long> > mp;     for (long long i = 1; i < n; i++) {       diff.push_back(v1[i] - v1[i - 1]);       mp[diff[i - 1]].push_back(i);     }     if (mp.size() > 3) {       cout << -1;       return 0;     }     if (mp.size() == 1) {       for (long long i = 0; i < n; i++) {         if (v[i] == v1[0]) {           cout << i + 1;           return 0;         }       }     }     if (mp.size() == 2) {       auto it = mp.begin();       vector<long long> v2 = it->second;       if (v2.size() == 1) {         long long x = v2[0], y;         if (x == 1) y = v1[0];         if (x == n - 1) y = v1[x];         if (x == 1 || x == n - 1) {           for (long long i = 0; i < n; i++) {             if (v[i] == y) {               cout << i + 1;               return 0;             }           }         }         if (it->first == 0) {           long long y = v1[v2[0]];           for (long long i = 0; i < n; i++) {             if (v[i] == y) {               cout << i + 1;               return 0;             }           }         }       }       if (v2.size() == 2) {         auto it2 = it;         it2++;         if (v2[1] - v2[0] == 1 && it->first * 2 == it2->first) {           long long x = v2[0], y;           y = v1[x];           for (long long i = 0; i < n; i++) {             if (v[i] == y) {               cout << i + 1;               return 0;             }           }         }       }       it++;       v2 = it->second;       if (v2.size() == 1) {         long long x = v2[0], y;         if (x == 1) y = v1[0];         if (x == n - 1) y = v1[x];         if (x == 1 || x == n - 1) {           for (long long i = 0; i < n; i++) {             if (v[i] == y) {               cout << i + 1;               return 0;             }           }         }       }       if (v2.size() == 2) {         auto it2 = it;         it2++;         if (v2[1] - v2[0] == 1 && it->first * 2 == it2->first) {           long long x = v2[0], y;           y = v1[x];           for (long long i = 0; i < n; i++) {             if (v[i] == y) {               cout << i + 1;               return 0;             }           }         }       }     }     if (mp.size() == 3) {       auto it = mp.begin();       auto it1 = it;       it1++;       auto it2 = it1;       it2++;       if (it->first + it1->first == it2->first) {         if (abs(it->second[0] - it1->second[0]) == 1) {           long long x1 = it->second[0], x2 = it1->second[0];           long long x = min(x1, x2);           long long y = v1[x];           for (long long i = 0; i < n; i++) {             if (v[i] == y) {               cout << i + 1;               return 0;             }           }         }       }     }     cout << -1;     return 0;   }