#include<bits/stdc++.h>
using namespace std;
int n,a[100005],l,r,m,x;
int main(){
    cin>>n;
    for(int i=1;i<=n;i++)cin>>a[i];
    cin>>x;
    l=1;
    r=n;
    while(l<=r){
    	m=(l+r)/2;
    	if(a[m]==x){
    		cout<<m;
    		return 0;
    	}
    	else if(a[m]<x)l=m+1;
    	else if(a[m>x])r=m-1;
    }
    cout<<-1;
	return 0;
}

/**************************************************************
	Problem: 1236
	User: chenjingqi
	Language: C++
	Result: Runtime Error
****************************************************************/