You are given an integer N. Consider all possible segments on the coordinate axis with endpoints at integer points with coordinates between 0 and N, inclusive; there will be <image> of them.

You want to draw these segments in several layers so that in each layer the segments don't overlap (they might touch at the endpoints though). You can not move the segments to a different location on the coordinate axis. 

Find the minimal number of layers you have to use for the given N.

Input

The only input line contains a single integer N (1 ≤ N ≤ 100).

Output

Output a single integer - the minimal number of layers required to draw the segments for the given N.

Examples

Input

2


Output

2


Input

3


Output

4


Input

4


Output

6

Note

As an example, here are the segments and their optimal arrangement into layers for N = 4.

<image>

import java.io.*;
import java.util.*;

public class Solving1 {

    public static void main(String[] args) throws IOException {
        FastScanner sc=new FastScanner(System.in);
        PrintWriter out=new PrintWriter(new BufferedWriter(new OutputStreamWriter(System.out)));
        int n=sc.nextInt()+1;
        
        int a[]=new int[n+1];
        
        for(int i=1;i<=n;i++){
            for(int j=1;j+i<=n;j++){
                inc(a,j,i+j);
            }
            
        }
        int ans=0;
        for(int i:a)ans=Math.max(ans, i);
        System.out.println(ans);
        
    }
    public static void inc(int a[],int s,int e){
        for(int i=s;i<e;i++){
            a[i]++;
        }
    }
    
  //__________________________________________________________
    static class FastScanner {
	    BufferedReader br;
	    StringTokenizer st;
            public FastScanner(InputStream i) {
	        br =  new BufferedReader(new InputStreamReader(i));
	        st = new StringTokenizer("");
	    }
	    public String next() throws IOException {
	        if(st.hasMoreTokens())return st.nextToken();
                else st = new StringTokenizer(br.readLine());
	        return next();
	    }
	    public int nextInt() throws IOException {return Integer.parseInt(next());}
	    public long nextLong() throws IOException {return Long.parseLong(next());}
	    public double nextDouble() throws IOException {return Double.parseDouble(next());}
	}
}
 