C++ :

#include<bits/stdc++.h>
using namespace std;

int n,i,p,x;
int a[1000100];
int main() {
	cin>>n;
	for(int i = 0;i < n;i++){
		cin>>a[i];
	}
	cin>>x;
	
	//二分在[l,r)之间查找左边界 
	int l = 0,r = n,mid;
	while(l < r){
		mid = l + (r - l) / 2;
		if(x == a[mid]) l = mid + 1;//继续向右找
		else if(x > a[mid]) l = mid + 1; 
		else if(x < a[mid]) r = mid;
	}
	
	if(a[l-1] == x) cout<<l;
	else cout<<-1;
}