#include<bits/stdc++.h>
using namespace std;
long long a[100001];
long long n,x;
long long ans;
bool f=false;
int main(){
	cin>>n;
    for(int i=1;i<=n;i++){
    	cin>>a[i];
    }
    cin>>x;
    long long left=1,middle,right=n;
    while(left<=right){
    	middle=(left+right)/2;
    	if(a[middle]==x){
    		ans==middle;
    		f=true;
    		break;
    	}
    	else if(x<a[middle]){
    		right=middle-1;
    	}
    	else if(x<a[middle]){
    		left=middle+1;
    	}
    }
    if(f==true){
    	cout<<ans;
    }
    else{
    	cout<<"-1"<<endl;
    }
    return 0;
}
/**************************************************************
	Problem: 1236
	User: zengdongxin
	Language: C++
	Result: Runtime Error
****************************************************************/