#include <iostream>
#include <vector>

int main(int argc, char const *argv[])
{
  std::ios_base::sync_with_stdio(false);

  int n;
  while (std::cin >> n)
  {
    std::vector<int> seq(n);
    for (int i = 0; i < n; ++i)
    {
      std::cin >> seq[i];
    }

    if (n <= 2)
    {
      std::cout << "1\n";
      continue;
    }

    int diff = seq[n - 1] - seq[n - 2];
    int index = 1;
    for (int i = n - 1; i > 0; --i)
    {
      if (seq[i] - seq[i - 1] != diff)
      {
        index = i + 1;
        break;
      }
    }

    std::cout << index << '\n';
  }

  return 0;
}