#include <iostream>

using namespace std;

int main()
{
	int N;
	while (cin >> N)
	{
		long* seq = new long[N];
		for (int n = 0; n < N; n++)
		{
			long numb;
			cin >> numb;
			seq[n] = numb;
		}

		if (N == 1)
		{
			cout << 1 << "\n";
			continue;
		}

		int start_i = N - 1;
		long diff = seq[N-1] - seq[N-2];
		for (int p = N - 2; p > 0; p--)
		{
			if (seq[p] - seq[p - 1] != diff)
			{
				break;
			}
			start_i--;
		}

		cout << start_i << "\n";
	}

	return 0;
}