#include<bits/stdc++.h>
using namespace std;
#define ll long long
const ll N=1e8+5;
ll a[N],n,k,l,r;
ll find(ll g){
	l=1,r=n;
	while(l<=r){
		ll mid=(l+r)/2;
		if(a[mid]<=g){
			l=mid+1;
		}
		else{
			r=mid-1;
		}
	}
	if(a[l-1]==g){
		return l-1;
	}
	else{
		return -1;
	}
}
int main(){
	cin>>n;
	for(int i=1;i<=n;i++){
		cin>>a[i];
	}
	cin>>k;
	cout<<find(k);
	return 0;
}