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

int n, x, a[10000];
int bs(int x) {
	int l = 0, r = n + 1;
	while(l + 1 < r) {
		int m = (l + r) / 2;
		if(a[m - 1] < x)
			l = m;
		else
			r = m;
	}
	if(a[l] == x)
		return l;
	else
		return -1;
}

int main() {
	scanf("%d", &n);
	for(int i = 0; i < n; i++)
		scanf("%d", &a[i]);
	cin >> x;
	cout << bs(x);
	return 0;
}