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.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.InputMismatchException;

public class B909{

	void solve()
	{
		int n = ni();
		int[] f = new int[n];
		for(int s=1;s<=n;s++)
			for(int i=0;i+s<=n;i++)
				for(int j=i,k=0;k<s;j++,k++)
					f[j]++;
		int max = 0;
		for(int x : f)
			max = Math.max(max, x);
		out.println(max);
		
	}
	
	public static void main(String[] args){new B909().run();}
	
	private byte[] bufferArray = new byte[1024];
	private int bufLength = 0;
	private int bufCurrent = 0;
	InputStream inputStream;
	PrintWriter out;
	
	public void run()
	{
		inputStream = System.in;
		out = new PrintWriter(System.out);
		solve();
		out.flush();
	}
	
	int nextByte()
	{
		if(bufLength == -1)
			throw new InputMismatchException();
		if(bufCurrent >= bufLength)
		{
			bufCurrent = 0;
			try
			{bufLength = inputStream.read(bufferArray);}
			catch(IOException e)
			{ throw new InputMismatchException();}
			if(bufLength <= 0)
				return -1;
		}
		return bufferArray[bufCurrent++];
	}
	
	boolean isSpaceChar(int x)	{return (x < 33 || x > 126);}
	
	boolean isDigit(int x)	{return (x >= '0' && x <= '9');}
	
	int nextNonSpace()
	{
		int x;
		while((x=nextByte()) != -1 && isSpaceChar(x));
		return x;
	}
	
	int ni()
	{
		long ans = nl();
		if (ans >= Integer.MIN_VALUE && ans <= Integer.MAX_VALUE)
			return (int)ans;
		throw new InputMismatchException();
	}
	
	long nl()
	{
		long ans = 0;
		boolean neg = false;
		int x = nextNonSpace();
		if(x == '-') 
		{
			neg = true;
			x = nextByte();
		}
		while(!isSpaceChar(x))
		{
			if(isDigit(x))
			{
				ans = ans * 10 + x -'0';
				x = nextByte();
			}
			else
				throw new InputMismatchException();
		}
		return neg ? -ans : ans;
	}
	
	String ns()
	{
		StringBuilder sb = new StringBuilder();
		int x = nextNonSpace();
		while(!isSpaceChar(x))
		{
			sb.append((char)x);
			x = nextByte();
		}
		return sb.toString();
	}
	
	char nc()	{ return (char)nextNonSpace();}
	
	double nd()	{ return (double)Double.parseDouble(ns()); }
	
	char[] ca()	{ return ns().toCharArray();}
	
	char[][] ca(int n)
	{
		char[][] ans = new char[n][];
		for(int i=0;i<n;i++)
			ans[i] = ca();
		return ans;
	}
	
	int[] ia(int n)
	{
		int[] ans = new int[n];
		for(int i=0;i<n;i++)
			ans[i] = ni();
		return ans;
	}
	
	void db(Object... o) {System.out.println(Arrays.deepToString(o));}
	
}
