#include<stdio.h>

int binarySearch(int arr[],int n,int target){
    int low=0,high=n-1;
    
    while(low<=high){
        int mid=low+(high-low)/2;
        
        
        if(arr[mid]==target)
        return mid;
        else if(arr[mid]<target)
        low=mid+1;
        else
        high=mid-1;
    }
    return -1;
}

int main(){
    int n;
    scanf("%d",&n);
    
    int arr[n];
    for(int i=0;i<n;i++)
    scanf("%d",&arr[i]);
    
    int target;
    scan("%d",&target);
    
    int result=binarySearch(arr,n,target);
    printf("%d\n",result);
    
    return 0;
}