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.*;


import java.io.*;
import java.lang.*;
import java.lang.reflect.Array;
import java.math.*;

public class cf1 
{
	static class pair implements Comparable<pair>
	 {
		Integer x,y;
		pair(int x,int y)
		{
			this.x=x;
			this.y=y;
		}
		public int compareTo(pair o)
		{
			if(x!=o.x)
				return x.compareTo(o.x);
			else
				return y.compareTo(o.y);
		}  
	}
	static long mod=1000000007;
	static boolean prime[];
	public static void main(String args[])
	{
		InputReader in=new InputReader(System.in);
		PrintWriter pw=new PrintWriter(System.out);
		int n=in.nextInt();
		int total=1;
		for(int i=2;i<=n;i++)
		{
			int temp=n;
			temp-=i;
			if(temp==0)
				total+=1;
			else if(temp<i)
				total+=temp+1;
			else
				total+=i;
		}
		pw.println(total);
		pw.flush();
	}
	public static int distance(int x,int y,int p,int q)
	{
		int a=(x-p)*(x-p);
		int b=(y-q)*(y-q);
		int dis=a+b;
		return dis;
	}
	public static int bnum(int num)
	{
		int ans=0;
		for(int i=31;i>=0;i--)
		{
			if((num&(1<<i))!=0)
			{
				ans+=Math.pow(10,i);
			}
		}
		return ans;
	}
	public static int bsdown(int a[],int item)
	{
		int low=0,high=a.length-1,ans=-1;
		while(low<=high)
		{
			int mid=low+(high-low)/2;
			if(a[mid]<=item)
			{
				ans=mid;
				low=mid+1;
			}
			else
				high=mid-1;
				
		}
		return ans;
	}
	public static long inverse(long num)
	{
		return fastpow(num,mod-2,mod);
	}
	public static int bsupper(int a[],int item)
	{
		int low=0,high=a.length-1,ans=-1;
		while(low<=high)
		{
			int mid=low+(high-low)/2;
			if(a[mid]>=item)
			{
				ans=mid;
				high=mid-1;
			}
			else
				low=mid+1;
				
		}
		return ans;
	}	
	public static boolean isprime(int n)
	{
		for(int i=2;i*i<=n;i++)
		{
			if(n%i==0)
				return false;
		}
		return true;
	}
	public static long fact(long a,long mod)
	{
		long ans=1;
		for(int i=1;i<=a;i++)
		{
			ans*=i;
			ans%=mod;
		}
		return ans%mod;
		
	}
	public static long ncr(long n,long r,long mod)
	{
		long nom=1,dom=1;
		nom=(fact(n,mod))%mod;
		dom=(fact(r,mod)*fact(n-r,mod))%mod;
		return (nom*fastpow(dom,mod-2,mod))%mod;
	}
	public static long gcd(long x, long y)
	{
		if(x==0)
			return y;
		if(y==0)
			return x;
		long r=0, a, b;
		a = (x > y) ? x : y; // a is greater number
		b = (x < y) ? x : y; // b is smaller number
		r = b;
		while(a % b != 0)
		{
			r = a % b;
			a = b;
			b = r;
		}
		return r;
	}
	public static void prime(int n)
	{
		prime[0]=true;
		prime[1]=true;
		for(int i=2;i*i<=n;i++)
		{
			if(prime[i])
				continue;
			for(int j=i;j*i<=n;j++)
			{
				prime[i*j]=true;
			}
		}
	}
	static class InputReader 
	{
		 
		private final InputStream stream;
		private final byte[] buf = new byte[8192];
		private int curChar, snumChars;
		private SpaceCharFilter filter;

		public InputReader(InputStream stream) 
		{
			this.stream = stream;
		}

		public int snext() 
		{
			if (snumChars == -1)
				throw new InputMismatchException();
			if (curChar >= snumChars) 
			{
				curChar = 0;
				try 
				{
					snumChars = stream.read(buf);
				}
				catch (IOException e) 
				{
					throw new InputMismatchException();
				}
				if (snumChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}

		public int nextInt()
		{
			int c = snext();
			while (isSpaceChar(c))
			{
				c = snext();
			}
			int sgn = 1;
			if (c == '-')
			{
				sgn = -1;
				c = snext();
			}
			int res = 0;
			do
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			}
			while (!isSpaceChar(c));
			return res * sgn;
		}

		public long nextLong() 
		{
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			int sgn = 1;
			if (c == '-') 
			{
				sgn = -1;
				c = snext();
			}
			long res = 0;
			do
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			}
			while (!isSpaceChar(c));
			return res * sgn;
		}

		public int[] nextIntArray(int n) 
		{
			int a[] = new int[n];
			for (int i = 0; i < n; i++) 
			{
				a[i] = nextInt();
			}
			return a;
		}

		public String readString() 
		{
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			StringBuilder res = new StringBuilder();
			do
			{
				res.appendCodePoint(c);
				c = snext();
			}
			while (!isSpaceChar(c));
			return res.toString();
		}

		public String nextLine()
		{
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			StringBuilder res = new StringBuilder();
			do 
			{
				res.appendCodePoint(c);
				c = snext();
			}
			while (!isEndOfLine(c));
			return res.toString();
		}

		public boolean isSpaceChar(int c) 
		{
			if (filter != null)
				return filter.isSpaceChar(c);
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		private boolean isEndOfLine(int c) 
		{
			return c == '\n' || c == '\r' || c == -1;
		}

		public interface SpaceCharFilter 
		{
			public boolean isSpaceChar(int ch);
		}
	}
	public static long fastpow(long a,long b,long mod)
	{
		long ans=1;
		while(b!=0)
		{
			if(b%2==1)
			{
				ans=(ans*a)%mod;
			}
			a=(a*a)%mod;
			b/=2;
		}
		return ans;
	}
}