#include <stdio.h>

int main()
{
	int N;

	while (scanf("%d", &N) != EOF)
	{
		long X[N+1];
		for (int i = 0; i < N; i++)
		{
			long pom;
			scanf("%ld", &pom);
			X[i] = pom;
		}

		if (N == 1)
			printf("%d\n", 1);
		else
		{
			long diff = X[N - 1] - X[N - 2];
			bool broke = false;
			
			for (int i = N - 2; i >= 1; i--)
			{
				if (X[i] - X[i - 1] != diff)
				{
					printf("%d\n", i + 1);
					broke = true;
					break;
				}
			}

			if (!broke)
				printf("1\n");
		}
	}

	return 0;
}