#include <stdio.h>
#include <iostream>
using namespace std;
#define N 10000
int mas[N];
int BS(int l, int r, int key) {
	int mid = 0;
	if (l >= r) return -1;
	mid = (l + r) / 2;
	if (mas[mid] == mas[key]) return mid;
	if (mas[l] == mas[key]) return l;
	if (mas[r] == mas[key]) return r;
	if (mas[mid] < mas[key]) return BS(mid + 1, r, key);
	if (mas[mid] > mas[key]) return BS(l, mid - 1, key);
	return -1;
}
int main() {
	int n;
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> mas[i];
	}
	cout << BS(0, n - 1, 3);
	return 0;
}