#include <iostream>
using namespace std;
int main() {
	int len;
	while (cin >> len) {
		long long int nums[len];

		for (int i = 0; i < len; i++) {
			cin >> nums[i];
		}
		bool hasDiff = false; bool found = false;
		long long int diff;
		for (int i = len - 1; i > 0; i--) {
			if (!hasDiff) {diff = nums[i] - nums[i - 1]; hasDiff=true;continue;}
			if (nums[i] - nums[i - 1] != diff) {
				cout << (i + 1) << '\n';
				found = true;
				break;
			}
			
		}
		if (!found) cout << "1\n";
	}

	return 0;
}