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 CodeForce;
import java.io.*;
import java.util.*;
/**
 *
 * @author prabhat
 */
public class R455D2A {


    static long[] a;
    static int[] S;
    static long[] sum;
    static int mod=1000000007;
   static int[][] dp;
   static boolean[][] isPalin;
   static int max1=5000+10;
   static int[][] g;
   static int ver;
   static int n;
   static boolean[] visit;
   static LinkedList<Pair> l[];
   static int ans=0;
   static int[] color;
   static LinkedList<Long> list[];
  public static void main(String[] args) throws Exception
   {
   	InputReader in = new InputReader(System.in);
   	PrintWriter pw=new PrintWriter(System.out);
     int n=in.ii();
     /* 1 2 2 1 for k=n*(n+1)/2=4 ,  for k=5 1 2 3 2 1 */
     int ans=0;
     for(int i=1;i<=n;i++)
     {
       ans+=2*i<=n?i:n-i+1;
     }
     pw.println(ans);
   /* String s1=in.is().trim();
     String s2=in.is();
    LinkedList<String> l=new LinkedList<>();
      for(int i=0;i<s1.length();i++)
      {
        l.add(s1.substring(0,i+1));
      }
      LinkedList<String> list=new LinkedList<>();
      for(int i=0;i<l.size();i++)
      {
        String s=l.get(i)+s2.charAt(0);
        list.add(s);
      }
      Collections.sort(list);
     // for(String s:list)pw.println(s);
     pw.println(list.get(0));*/
   	 pw.close();
   }
 static int length(long n)
 {
  int cnt=0;
  while(n>0)
  {
   cnt++;
   n>>=1;
  }
  return cnt;
 }
  static  void dfs(int i)
  {
	  if(visit[i])return ;
	  visit[i]=true;
	  for(int o:g[i])
		  if(!visit[o])
			  dfs(o);
		  
  }
  private static class DSU{
		int[] parent;
		int[] rank;
		int cnt;
		public DSU(int n){
			parent=new int[n];
			rank=new int[n];
			for(int i=0;i<n;i++){
				parent[i]=i;
				rank[i]=1;
			}
			cnt=n;
		}
		int find(int i){
	        while(parent[i] !=i){
	        	parent[i]=parent[parent[i]];
	        	i=parent[i];
	        }
	        return i;
	    }
		int Union(int x, int y){
	        int xset = find(x);
	        int yset = find(y);
	        if(xset!=yset)
	        	cnt--;
	        if(rank[xset]<rank[yset]){
	        	parent[xset] = yset;
		        rank[yset]+=rank[xset];
		        rank[xset]=0;
		        return yset;
	        }else{
	        	parent[yset]=xset;
	        	rank[xset]+=rank[yset];
	        	rank[yset]=0;
	        	return xset;
	        }
	    }
	}
  public static int[][] packU(int n, int[] from, int[] to, int max) {
	    
	    int[][] g = new int[n][];
	    int[] p = new int[n];
	    for (int i = 0; i < max; i++) p[from[i]]++;
	    for (int i = 0; i < max; i++) p[to[i]]++;
	    for (int i = 0; i < n; i++) g[i] = new int[p[i]];
	    for (int i = 0; i < max; i++) {
	        g[from[i]][--p[from[i]]] = to[i];
	        g[to[i]][--p[to[i]]] = from[i];
	    }
	    return g;
	}
	public static int[][] parents3(int[][] g, int root) {
	    int n = g.length;
	    int[] par = new int[n];
	    Arrays.fill(par, -1);
 
	    int[] depth = new int[n];
	    depth[0] = 0;
 
	    int[] q = new int[n];
	    q[0] = root;
	    for (int p = 0, r = 1; p < r; p++) {
	        int cur = q[p];
	        for (int nex : g[cur]) {
	            if (par[cur] != nex) {
	                q[r++] = nex;
	                par[nex] = cur;
	                depth[nex] = depth[cur] + 1;
	            }
	        }
	    }
	    return new int[][]{par, q, depth};
	}
	public static int lower_bound(int[] nums, int target) {
        int low = 0, high = nums.length - 1;
        while (low < high) {
            int mid = low + (high - low) / 2;
            if (nums[mid] < target)
                low = mid + 1;
            else
                high = mid;
        }
        return nums[low] == target ? low : -1;
    }
 
    public static int upper_bound(int[] nums, int target) {
        int low = 0, high = nums.length - 1;
        while (low < high) {
            int mid = low + (high + 1 - low) / 2;
            if (nums[mid] > target)
                high = mid - 1;
            else
                low = mid;
        }
        return nums[low] == target ? low : -1;
    }
  public static boolean palin(String s)
	{
	   int i=0;
	   int j=s.length()-1;
	   while(i<j)
	   {
		   if(s.charAt(i)==s.charAt(j))
		   {
			   i++;
			   j--;
		   }
		   else return false;     
	   }
	   return true;
	}
	
    static boolean CountPs(String s,int n)
       {
    	boolean b=false;
    	     char[] S=s.toCharArray();
	       int[][] dp=new int[n][n];
	       boolean[][] p=new boolean[n][n];
	       for(int i=0;i<n;i++)p[i][i]=true;
	     
	       for(int i=0;i<n-1;i++)
	       {
	    	   if(S[i]==S[i+1])
	    	   {
	    		p[i][i+1]=true;
	    		dp[i][i+1]=1;
	    		b=true;
	    	   }
	       }
	    
	       for(int gap=2;gap<n;gap++)
	       {
	    	   for(int i=0;i<n-gap;i++)
	    	   {
	    		   int j=gap+i;
	    		   if(S[i]==S[j]&&p[i+1][j-1]){p[i][j]=true;}
	    		  
	    		  if(p[i][j])
	    			   dp[i][j]=dp[i][j-1]+dp[i+1][j]+1-dp[i+1][j-1];
	    		   
	    		   else  dp[i][j]=dp[i][j-1]+dp[i+1][j]-dp[i+1][j-1];
	    		  
	    		   if(dp[i][j]>=1){b=true;}
	    	   }
	       }
	      
	    return b; 
   // return dp[0][n-1];
	       
    }
  
  
  
 
 
public static int gcd(int a,int b)
{
	int res=1;
	while(a>0)
	{
		res=a;
		a=b%a;
		b=res;
	}
	return res;
}
 
 
public static long pow(long n,long p,long m)
{
	 long  result = 1;
	  if(p==0)
	    return 1;
	if (p==1)
	    return n;
	while(p!=0)
	{
	    if(p%2==1)
	        result *= n;
	    if(result>=m)
	    result%=m;
	    p >>=1;
	    n*=n;
	    if(n>=m)
	    n%=m;
	}
	return result;
}
public static long pow(long n,long p)
{
	long  result = 1;
	  if(p==0)
	    return 1;
	if (p==1)
	    return n;
	while(p!=0)
	{
	    if(p%2==1)
	        result *= n;
	    p >>=1;
	    n*=n;
	}
	return result;
 
}
 
 
 
 
static class Edge implements Comparator<Edge> {
	private int u;
	private int v;
	private int w;
	public Edge() {
	}
	public Edge(int u, int v, int w) {
		this.u=u;
		this.v=v;
		this.w=w;
	}
	public int getU() {
		return u;
	}
	public void setU(int u) {
		this.u = u;
	}
	public int getV() {
		return v;
	}
	public void setV(int v) {
		this.v = v;
	}
	public int getW() {
		return w;
	}
	public void setW(int w) {
		this.w = w;
	      }
	public int compareTo(Edge e)
	  {
		return this.getW() - e.getW();
	}
	@Override
	public String toString() {
		return "Edge [u=" + u + ", v=" + v + ", w=" + w + "]";
	}
	@Override
	public int compare(Edge arg0, Edge arg1) {
		// TODO Auto-generated method stub
		return 0;
	}
}
 
 
 
static class Pair implements Comparable<Pair>
{
	int a,b;
	Pair (int a,int b)
	{
		this.a=a;
		this.b=b;
	}
 
	public int compareTo(Pair o) {
		// TODO Auto-generated method stub
		if(this.a!=o.a)
		return -Integer.compare(this.a,o.a);
		else
			return -Integer.compare(this.b, o.b);
		//return 0;
	}
	public boolean equals(Object o) {
       if (o instanceof Pair) {
           Pair p = (Pair)o;
           return p.a == a && p.b == b;
       }
       return false;
   }
   public int hashCode() {
       return new Integer(a).hashCode() * 31 + new Integer(b).hashCode();
   }
}
 
static  class InputReader
{
   private InputStream stream;
   private byte[] buf = new byte[1024];
 
   private SpaceCharFilter filter;
    byte inbuffer[] = new byte[1024];
   int lenbuffer = 0, ptrbuffer = 0;
    final int M = (int) 1e9 + 7;
   int md=(int)(1e7+1);
    int[] SMP=new int[md];
   final double eps = 1e-6;
   final double pi = Math.PI;
 
	PrintWriter out;
   String check = "";
 InputStream   obj = check.isEmpty() ? System.in : new ByteArrayInputStream(check.getBytes());
   public InputReader(InputStream stream)
   {
       this.stream = stream;
   }
 
    int readByte() {
	        if (lenbuffer == -1) throw new InputMismatchException();
	        if (ptrbuffer >= lenbuffer) {
	            ptrbuffer = 0;
	            try {
	                lenbuffer = obj.read(inbuffer);
	            } catch (IOException e) {
	                throw new InputMismatchException();
	            }
	        }
	        if (lenbuffer <= 0) return -1;
	        return inbuffer[ptrbuffer++];
   }
 
   public  int read()
   {
       int c = read();
       while (isSpaceChar(c))
           c = read();
       int sgn = 1;
       if (c == '-')
       {
           sgn = -1;
           c = read();
       }
       int res = 0;
       do
       {
           if (c < '0' || c > '9')
               throw new InputMismatchException();
           res *= 10;
           res += c - '0';
           c = read();
       } while (!isSpaceChar(c));
       return res * sgn;
   }
 
   String is() {
	        int b = skip();
	        StringBuilder sb = new StringBuilder();
	        while (!(isSpaceChar(b))) // when nextLine, (isSpaceChar(b) && b!=' ')
	        {
	            sb.appendCodePoint(b);
	            b = readByte();
	        }
	        return sb.toString();
   }
   public   int ii() {
	        int num = 0, b;
	        boolean minus = false;
	        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')) ;
	        if (b == '-') {
	            minus = true;
	            b = readByte();
	        }
	        while (true) {
	            if (b >= '0' && b <= '9') {
	                num = num * 10 + (b - '0');
	            } else {
	                return minus ? -num : num;
	            }
	            b = readByte();
	        }
   }
 public   long il() {
	        long num = 0;
	        int b;
	        boolean minus = false;
	        while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-')) ;
	        if (b == '-') {
	            minus = true;
	            b = readByte();
	        }
	        while (true) {
	            if (b >= '0' && b <= '9') {
	                num = num * 10 + (b - '0');
	            } else {
	                return minus ? -num : num;
	            }
	            b = readByte();
	        }
   }
   
	    boolean isSpaceChar(int c) {
	        return (!(c >= 33 && c <= 126));
	       }
 
	      int skip()
	        {
	        int b;
	        while ((b = readByte()) != -1 && isSpaceChar(b)) ;
	        return b;
	        }
 
	    float nf() {
	        return Float.parseFloat(is());
	    }
 
	    double id() {
	        return Double.parseDouble(is());
	    }
 
	    char ic() {
	        return (char) skip();
	    }
 
	    int[] iia(int n) {
	        int a[] = new int[n];
	        for (int i = 0; i<n; i++) a[i] = ii();
	        return a;
	    }
 
	    long[] ila(int n) {
	        long a[] = new long[n];
	        for (int i = 0; i <n; i++) a[i] = il();
	        return a;
	    }
 
	    String[] isa(int n) {
	        String a[] = new String[n];
	        for (int i = 0; i < n; i++) a[i] = is();
	        return a;
	    }
 
 
		 long mul(long a, long b) { return a * b % M; }
 
			long div(long a, long b)
				 {
					return mul(a, pow(b, M - 2));
 
		           }
 
 
 
	    double[][] idm(int n, int m) {
	        double a[][] = new double[n][m];
	        for (int i = 0; i < n; i++) for (int j = 0; j < m; j++) a[i][j] = id();
	        return a;
	        }
 
	    int[][] iim(int n, int m) {
	        int a[][] = new int[n][m];
	        for (int i = 0; i < n; i++) for (int j = 0; j <m; j++) a[i][j] = ii();
	        return a;
          }
 
 
   public String readLine() {
       int c = read();
       while (isSpaceChar(c))
           c = read();
       StringBuilder res = new StringBuilder();
       do {
           res.appendCodePoint(c);
           c = read();
       } while (!isEndOfLine(c));
       return res.toString();
   }
 
   public interface SpaceCharFilter
   {
       public boolean isSpaceChar(int ch);
   }
   public boolean isEndOfLine(int c) {
       return c == '\n' || c == '\r' || c == -1;
   }
}
 
}
 