#include <bits/stdc++.h>

using namespace std;\

int main() {
	int n, old_diff = 0, diff = 0;
	ios::sync_with_stdio(false);
	while(scanf("%d", &n) == 1) {
		int num1, num2, res = 1;
		scanf("%d ", &num1);
		if (n == 1) {
			cout << "1" << endl;
			continue;
		}	
		scanf("%d ", &num2);
		if (n == 2) {
			cout << "1" << endl;
			continue;
		}
		old_diff = num2-num1;
		num1= num2;
		for(int i = 3; i <= n; i++) {
			scanf("%d ", &num2);
			diff = num2 - num1; 
			//cout << old_diff << " "<< diff<< endl; 
			if (old_diff != diff) {
				res = i-1; 
				old_diff = diff;
			}
			num1 = num2;
		}
		cout << res << endl;
	}
	return 0;
}