#include <bits/stdc++.h>
using namespace std;

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

		int diff = arr[n - 1] - arr[n - 2];
		for(int i = n-2; i >= 0; i--) {
			if(arr[i] - arr[i - 1] != diff) {
				printf("%d\n", i + 1);
				break;
			}
		}
	}
	
}