#include<cstdio>
#include<iostream>
#include<algorithm>
using namespace std;
const int N=1e6+10;
int a[N];
int main(){
int n,x;
cin>> n;
for(int i=1;i<=n;i++){
cin >> a[i];
}
cin >>x;
int l=1,r=n;
while(l<=r){
int mid =(l+r)/2;
if(a[mid]==x){
cout<<mid;
return 0;
}
else if(a[mid]>x){
r=mid-1;
}
else{
l=mid+1;
}
}
cout << -1;
return 0;
}