#include<bits/stdc++.h>
using namespace std;
int n;
int b[10004];
int yf(int a[],int n,int x){
    int l=0;
    int r=n-1;
    while(l<=r){
        int mid=(l+r)/2;
        if(a[mid]<x)l=mid+1;
        else r=mid-1; 
    }
    if(a[l]==x)return l;
    else return -1;
}
int main(){
    cin>>n;
    for(int i=1;i<=n;i++){
            cin>>b[i];    
    }
    int y;
    cin>>y;
    cout<<yf(b,n,y);
}