#include <iostream>

using namespace std;

int search(int num[],int start,int final,int x);

int main()
{	

	int n[20];
	for(int i=0;i<20;i++)
	{
		n[i]=i+1;
		cout<<n[i]<<endl;
	}
	
	int x;
	cout<<"input a number : ";
	
	while(cin>>x)
		cout<<search(n,0,20,x)<<endl;			
}

int search(int num[],int start,int final,int x)
{	
	if(start>final)
		return -1;
		
	else
	{
		int mid=(start+final)/2;
		if(num[mid]==x)
			return mid;
		else if(num[mid]>x)
			return search(num,mid+1,final,x);
		else if(num[mid]<x)
			return search(num,start,mid-1,x);
	}
}