func_code_string: #include <bits/stdc++.h>   using namespace std;   int n;   vector<pair<int, int>> vp;   int chk() {     bool ok = true;     int d = vp[2].first - vp[1].first;     for (int i = 2; i < n; i++)       if (vp[i].first - vp[i - 1].first != d) ok = false;     if (ok) {       printf( %d n , vp[0].second + 1);       exit(0);     }     d = vp[2].first - vp[0].first;     ok = true;     for (int i = 3; i < n; ++i)       if (vp[i].first - vp[i - 1].first != d) ok = false;     if (ok) {       printf( %d n , vp[1].second + 1);       exit(0);     }     d = vp[1].first - vp[0].first;     int cnt = 0, index = 0;     ok = true;     for (int i = 1; i < n; i++) {       if (vp[i].first - vp[i - 1].first == d) continue;       cnt++;       if (i == n - 1 or vp[i + 1].first - vp[i - 1].first != d) ok = false;       index = vp[i].second;       i++;     }     if (ok and cnt == 1) {       cout << index + 1 << endl;       exit(0);     }   }   int main() {     scanf( %d , &n);     vp.resize(n);     for (int i = 0; i < n; ++i) {       scanf( %d , &vp[i].first);       vp[i].second = i;     }     sort(begin(vp), end(vp));     chk();     if (n < 4) return !printf( 1 n );     reverse(begin(vp), end(vp));     chk();     cout << -1 << endl;   }   