#include <bits/stdc++.h>   using namespace std;   long long int power(long long int a, long long int b) {     if (b == 0) return 1;     if (b == 1) return a;     long long int r = power(a, b / 2) % 1000000007;     if (b % 2 == 0)       return (r * r) % 1000000007;     else       return (((r * r) % 1000000007) * a) % 1000000007;   }   int prime[1000009];   void sieve(int n) {     prime[0] = prime[1] = 1;     for (int i = 2; i <= sqrt(n); i++) {       if (!prime[i]) {         for (int j = 2 * i; j <= n; j++) {           prime[j]++;         }       }     }   }   bool comp(const pair<long long int, long long int> &a,             const pair<long long int, long long int> &b) {     return (a.second < b.second);   }   const int N = 5 * 1e5 + 9;   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     cout.tie(NULL);     int n;     cin >> n;     long long int a[n];     vector<pair<long long int, int> > v;     for (int i = 0; i < n; i++) {       cin >> a[i];       v.push_back({a[i], i + 1});     }     sort(v.begin(), v.end());     if (n == 2 || n == 3) {       cout <<  1 n ;       return 0;     }     map<long long int, long long int> mp;     for (int i = 0; i < n - 1; i++) {       mp[v[i + 1].first - v[i].first]++;     }     if (mp.size() > 3) {       cout <<  -1 n ;       return 0;     }     if (mp.size() == 1) {       cout << v[0].second <<   n ;       return 0;     }     if (n == 4) {       if (v[1].first + v[3].first == 2 * v[2].first) {         cout << v[0].second <<   n ;         return 0;       }       if (v[0].first + v[3].first == 2 * v[2].first) {         cout << v[1].second <<   n ;         return 0;       }       if (v[0].first + v[3].first == 2 * v[1].first) {         cout << v[2].second <<   n ;         return 0;       }       if (v[0].first + v[2].first == 2 * v[1].first) {         cout << v[3].second <<   n ;         return 0;       }       cout <<  -1 n ;       return 0;     }     for (int i = 0; i < n; i++) {       if (i == 0) {         mp[v[i + 1].first - v[i].first]--;         if (mp[v[i + 1].first - v[i].first] == 0) {           mp.erase(v[i + 1].first - v[i].first);         }         if (mp.size() == 1) {           cout << v[0].second <<   n ;           return 0;         }         mp[v[i + 1].first - v[i].first]++;       } else if (i == n - 1) {         mp[v[i].first - v[i - 1].first]--;         if (mp[v[i].first - v[i - 1].first] == 0) {           mp.erase(v[i].first - v[i - 1].first);         }         if (mp.size() == 1) {           cout << v[n - 1].second <<   n ;           return 0;         }         mp[v[i].first - v[i - 1].first]++;       } else {         mp[v[i + 1].first - v[i].first]--;         mp[v[i].first - v[i - 1].first]--;         if (mp[v[i + 1].first - v[i].first] == 0) {           mp.erase(v[i + 1].first - v[i].first);         }         if (mp[v[i].first - v[i - 1].first] == 0) {           mp.erase(v[i].first - v[i - 1].first);         }         mp[v[i + 1].first - v[i - 1].first]++;         if (mp.size() == 1) {           cout << v[i].second <<   n ;           return 0;         }         mp[v[i + 1].first - v[i - 1].first]--;         if (mp[v[i + 1].first - v[i - 1].first] == 0) {           mp.erase(v[i + 1].first - v[i - 1].first);         }         mp[v[i + 1].first - v[i].first]++;         mp[v[i].first - v[i - 1].first]++;       }     }     cout <<  -1 n ;     return 0;   }