#include<bits/stdc++.h>
using namespace std;
int n,a[1000005],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: zhengzihao
	Language: C++
	Result: Accepted
	Time:297 ms
	Memory:5980 kb
****************************************************************/