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>
//package CF_455;

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

public class b {
	public static void main(String[] args) throws Exception{ new b(new Reader()); }
	public b(Reader rr) throws IOException{
		int n=rr.ni()+1;
		System.out.println((int)Math.floor(n*n/4));
	}
	static class Reader{
		private DataInputStream din;
		private byte[] buffer=new byte[1024];
		private int bufP, bytR;
		public Reader() throws IOException{
			din=new DataInputStream(System.in);
			bufP=bytR=0;
		}
		public Reader(String file) throws IOException{
			din=new DataInputStream(new FileInputStream(file));
			bufP=bytR=0;
		}
		private String rl() throws IOException{
			byte[] buf=new byte[1024];
			int cnt=0, c;
			while((c=read())!=-1){
				if(c=='\n') break;
				buf[cnt++]=(byte)c;
			}
			return new String(buf, 0, cnt);
		}
		private int ni() throws IOException{
			int num=0;
			byte c=read();
			while(c<=' ') c=read();
			boolean neg=(c=='-');
			if(neg) c=read();
			do{
				num=num*10+c-'0';
			} while((c=read())>='0'&&c<='9');
			if(neg) return -num;
			return num;
		}
		private long nl() throws IOException{
			long num=0;
			byte c=read();
			while(c<=' ') c=read();
			boolean neg=(c=='-');
			if(neg) c=read();
			do{
				num=num*10+c-'0';
			} while((c=read())>='0'&&c<='9');
			if(neg) return -num;
			return num;
		}
		private double nd() throws IOException{ return Double.parseDouble(ns()); }
		private char nc() throws IOException{ return (char)next(); }
		private String ns() throws IOException{
			int c=next();
			StringBuilder sb=new StringBuilder();
			while(!(isChar(c))){
				sb.appendCodePoint(c);
				c=read();
			}
			return sb.toString();
		}
		private char[] ns(int n) throws IOException{
			char[] buf=new char[n];
			int c=next(), r=0;
			while(r<n&&!(isChar(c))){
				buf[r++]=(char)c;
				c=next();
			}
			return n==r?buf:Arrays.copyOf(buf, r);
		}
		private char[][] nm(int n, int m) throws IOException{
			char[][] map=new char[n][];
			for(int i=0; i<n; i++) map[i]=ns(m);
			return map;
		}
		private int[] na(int n) throws IOException{
			int[] a=new int[n];
			for(int i=0; i<n; i++) a[i]=ni();
			return a;
		}
		private boolean isChar(int c) throws IOException{ return !(c>=33&&c<=126); }
		private int next() throws IOException{ int c; while((c=read())!=-1&&isChar(c)); return c; }
		private byte read() throws IOException{
			if(bufP==bytR){
				bytR=din.read(buffer, bufP=0, 1024);
				if(bytR==-1) buffer[0]=-1;
			}
			return buffer[bufP++];
		}
		public void close() throws IOException{
			if(din==null) return;
			din.close();
		}
	}
}
