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>
/* 
 * UMANG PANCHAL
 * DAIICT
 */
import java.util.*;
import java.io.*;
import java.math.BigInteger;
public class Main
{
 
	static LinkedList<Integer> adj[];
	static ArrayList<Integer> adj1[];
	static int[] color,visited1;
	static boolean b[],visited[],possible;
	static int level[];
	static Map<Integer,HashSet<Integer>> s;
	static int totalnodes,colored;
	static int count[];
	static long sum[];
	static int nodes;
	static double ans=0;
	static long[] as=new long[10001];
	static long c1=0,c2=0;
	static int[] a,d,k;
	static int max=100000000;
	static long MOD = (long)1e9 + 7,sm=0,m=Long.MIN_VALUE;
	static boolean[] prime=new boolean[1000005];
	static int[] levl;
    static int[] eat;
   // static int n,m;
    static int price[];
    static int size[],res[],par[];
    //ind frog crab
static int result=0;
	// --------------------My Code Starts Here----------------------
	
	public static void main(String[] args) throws IOException
	{
		in=new InputReader(System.in);
		w=new PrintWriter(System.out);
        int n=ni();
        w.print(ans(n));
       
       w.close();
    }
	
	public static int ans(int n)
	{
		if(n==1)
			return 1;
		else if(n==2)
			return 2;
		else
			return n+ans(n-2);
	}
	public static long fact(long n)
	{
		long p=n%MOD;
		for(long i=n-1;i>=1;i--)
			p=(p*(i%MOD))%MOD;
		return p%MOD;
	}
	public static long nCrModp(long n, long r, long p)
	{
	    
	    long[] C=new long[(int)r+1];
	    
	 
	    C[0] = 1; 
	    
	    for (long i = 1; i <= n; i++)
	    {
	        
	        for (long j = Math.min(i, r); j > 0; j--)
	 
	            
	            C[(int)j] = (C[(int)j] + C[(int)(j-1)])%p;
	    }
	    return C[(int)r];
	}
	public static long nCr(long n, long r)
	{
		long x=1;
		for(long i=n;i>=n-r+1;i--)
			x=((x)*(i));
		for(long i=r;i>=1;i--)
			x=((x)/(i));
		
		return x%MOD;
	}
	public static long nCrModpDP(long n, long r, long p)
	{
	
	    long[] C=new long[(int)r+1];
	 
	    C[0] = 1; 
	 
	 
	    for (long i = 1; i <= n; i++)
	    {
	        for (int j = (int)Math.min(i, r); j > 0; j--)
	           C[j] = (C[j] + C[j-1])%p;
	    }
	    return C[(int)r];
	}

	public static long nCrModpLucas(long n,long r, long p)
	{
	  
	   if (r==0)
	      return 1;
	 
	   long ni = n%p, ri = r%p;
	
	   return (nCrModpLucas(n/p,r/p, p) * nCrModpDP(ni,ri, p)) % p;  
	}
	
	 public static void buildgraph(int n){
         adj=new LinkedList[n+1];
         visited=new boolean[n+1];
         levl=new int[n+1];
         par=new int[n+1];
         for(int i=0;i<=n;i++){
             adj[i]=new LinkedList<Integer>();
         
         }


     }
	public static int getSum(long BITree[], int index)
	{
	    int sum = 0;
	    
	    while (index > 0)
	    {
	        sum += BITree[index];
	        index -= index & (-index);
	    }
	    return sum;
	}
	
	public static long[] updateBIT(long BITree[], int n, int index, int val)
	{
	    while (index <= n)
	    {
	       BITree[index] += val;
	       index += index & (-index);
	    }
	    return BITree;
	}
	

	
	public static boolean isPrime(int n)
	{
	    // Corner cases
	    if (n <= 1)  return false;
	    if (n <= 3)  return true;
	 
	    // This is checked so that we can skip 
	    // middle five numbers in below loop
	    if (n%2 == 0 || n%3 == 0)
	    	return false;
	 
	    for (int i=5; i*i<=n; i=i+6)
	        if (n%i == 0 || n%(i+2) == 0)
	           return false;
	 
	    return true;
	}
	
	// --------------------My Code Ends Here------------------------
	
	public static String ns()
	{
		return in.nextLine();
	}
	public static int ni()
	{
		return in.nextInt();
	}
	public static long nl()
	{
		return in.nextLong();
	}
	public static int[] na(int n)
	{
		a=new int[n];
		for(int i=0;i<n;i++)
		a[i]=ni();
		return a;
	}
	public static void sieveOfEratosthenes()
	    {
	       int n=prime.length;
	        for(int i=0;i<n;i++)
	            prime[i] = true;
	         
	        for(int p = 2; p*p <=n; p++)
	        {
	            if(prime[p] == true)
	            {
	                for(int i = p*2; i <n; i += p)
	                    prime[i] = false;
	            }
	        }
	    }

	public static boolean printDivisors(long n)
    {
		long ans=0;
        for (int i=1; i<=Math.sqrt(n); i++)
        {
            if (n%i==0)
            {
                if (n/i == i)
                    ans++;
                else 
                {
                	ans=ans+2;
                }
            }
            if(ans>3)
            	break;
        }
        if(ans==3)
        	return true;
        else
        	return false;
    }

  public static void dfs(int i)
  {
	  visited[i]=true;
	  for(int j:adj[i])
	  {
		  if(!visited[j])
			  {
			  dfs(j);
			  nodes++;
			  }
	  }
	  
  }
  public static String rev(String s)
	{
		StringBuilder sb=new StringBuilder(s);
		sb.reverse();
		return sb.toString();
	}
	static long lcm(long a, long b)
	{
	    return a * (b / gcd(a, b));
	}
	static long gcd(long a, long b)
	{
	    while (b > 0)
	    {
	        long temp = b;
	        b = a % b; // % is remainder
	        a = temp;
	    }
	    return a;
	}
	static InputReader in;
	static PrintWriter w;
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 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);
}
}
	
}