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>

    //~~~~~~~~~~~~~~~~~~~~~~~@@@@@@@@@@@@@@@_____________K_____S_____J__________@@@@@@@@@@@@@@@@@@@@@@@@@@@@~~~~~~~~~~~~~~~~~~~~~~~~~~
    //Date:00/00/17
    //-------------


import java.util.*;
import java.io.*;

/*

    [[[[[[[[[[[[[[[      ]]]]]]]]]]]]]]]
    [::::::::::::::      ::::::::::::::]
    [::::::::::::::      ::::::::::::::]
    [::::::[[[[[[[:      :]]]]]]]::::::]
    [:::::[                      ]:::::]
    [:::::[                      ]:::::]
    [:::::[                      ]:::::]
    [:::::[                      ]:::::]
    [:::::[    CODE YOUR LIFE    ]:::::]
    [:::::[   Kripa Shankar jha  ]:::::]
    [:::::[                      ]:::::]
    [:::::[                      ]:::::]
    [:::::[                      ]:::::]
    [:::::[                      ]:::::]
    [::::::[[[[[[[:      :]]]]]]]::::::]
    [::::::::::::::      ::::::::::::::]
    [::::::::::::::      ::::::::::::::]
    [[[[[[[[[[[[[[[      ]]]]]]]]]]]]]]]
*/

public class prob3{
  

    static Parser sc=new Parser(System.in);
    static PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out));
  
  static int p[]=new int[100005];
  public static void main(String[] args) throws IOException {
    // use ((((((( sc ............... for input

    int n=sc.nextInt();

    int ans=0,t=n/2;
    
    ans=(t*(t+1));

    if(n%2!=0){

      ans+=(t+1);
    }

    System.out.println(""+ans);    
        
  }

   

public static void union(int a,int b){
        int i=find(a);
        int j=find(b);

        if(p[i]!=j){
          p[i]=j;
        }
      }


      public static int find(int a){

        while(p[a]!=a){
          a=p[a];
        }

        return a;
      }






  //___________________________Fast-Input_Output-------------------******************* 
  

  static class Parser {
        final private int BUFFER_SIZE = 1 << 20;  // 2^16, a good compromise for some problems
        private InputStream din;    // Underlying input stream
        private byte[] buffer;      // Self-maintained buffer
        private int bufferPointer;  // Current read position in the buffer
        private int bytesRead;      // Effective bytes in the buffer read from the input stream
        private SpaceCharFilter filter;
        public Parser(InputStream in) {
            din = in;
            buffer = new byte[BUFFER_SIZE];
            bufferPointer = bytesRead = 0;

        }

        /**
         * Read the next integer from the input stream.
         * @return The next integer.
         * @throws IOException
         */
        public int nextInt() throws IOException {
            int result = 0;
            byte c = read();
            while (c <= ' ') c = read();
            boolean neg = (c == '-');
            if (neg) c = read();
        while (c >= '0' && c <= '9') {
                result = result * 10 + c - '0';
                c = read();
            }
      if (neg) return -result;
              return result;
        }

        public int nextLong() throws IOException {
            int result = 0;
            byte c = read();
            while (c <= ' ') c = read();
            boolean neg = (c == '-');
            if (neg) c = read();
        while (c >= '0' && c <= '9') {
                result = result * 10 + c - '0';
                c = read();
            }
      if (neg) return -result;
              return result;
        }

        public String nextLine()throws IOException {
        int c = read();
        while (isSpaceChar(c))
          c = read();
        StringBuilder res = new StringBuilder();
        do {
          res.appendCodePoint(c);
          c = read();
        } while (!isEndOfLine(c)==true);
        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);
      }


        /**
         * Read the next byte of data from the input stream.
         * @return the next byte of data, or -1 if the end of the stream is reached.
         * @throws IOException if an I/O error occurs.
         */
        public byte read() throws IOException {
            if (bufferPointer == bytesRead)
                fillBuffer();
            return buffer[bufferPointer++];
        }

        /**
         * Read data from the input stream into the buffer
         * @throws IOException if an I/O error occurs.
         */
        private void fillBuffer() throws IOException {
            bytesRead = din.read(buffer, bufferPointer = 0, BUFFER_SIZE);
            if (bytesRead == -1)
                buffer[0] = -1;
        }
    }
}