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.util.*;


public class test
{	
	/*
	private static class IntegerPair implements Comparable
	{
	    public Integer first;
	    public Integer second;

	    public IntegerPair(Integer f, Integer s) {
			first = f;
			second = s;
	    }

		public int compareTo(Object obj) 
		{
			if (!this.first.equals( ((IntegerPair)obj).first) )
			{
			    return first - ((IntegerPair)obj).first;
			}
			else
			{
			    return second - ((IntegerPair)obj).second;
			}
		}
	}
	*/
	
	/*
	//Arrays.sort(data, new MyComparator());
	//if a<b return -1
	//if a>b return 1
	//if a==b return 0
	private static class MyComparator implements Comparator<Long> 
	{
		@Override
		public int compare(Long a, Long b)
		{
			int a3 = 0;
			int b3 = 0;
			
			while(a%3==0)
			{
				a/=3;
				a3++;
			}
			while(b%3==0)
			{
				b/=3;
				b3++;
			}
			
			if(a3>b3)
			{
				return -1;
			}
			else if(a3 == b3 && a<b)
			{
				return -1;
			}
			return 1;
			
		}
	}
	*/


    private static Scanner sc = new Scanner(System.in);
	//private static String abc[] = {"a","b","c","d","e","f","g","h","i","j","k","l","m","n","o","p","q","r","s","t","u","v","w","x","y","z"};
	


	
	
	
	
	

	
	public static void main(String[] args)
    {
		int n = sc.nextInt();
		
		int cnt[] = new int[n+10];
		for(int start = 1;start <= n;start++)
		{
			for(int end = start;end<=n;end++)
			{
				cnt[start]++;
				cnt[end+1]--;
			}
		}
		int answer = 0;
		for(int i=1;i<=n;i++)
		{
			cnt[i]+=cnt[i-1];
			
			if(cnt[i] > answer)answer = cnt[i];
		}
		System.out.print(answer);


			
			

		
		

		
	}
	
	
	

}





/*
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9  12
10 19
11 39
12 67

13 69
14 68
15 74
*/


