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>
// Magic. Do not touch.
import java.io.*;
import java.math.*;
import java.util.*;

public class Main {
    static int mod=(int)(1e9+7);
    static class FastReader
    {
        PrintWriter pw = new PrintWriter(System.out);
        private InputStream mIs;private byte[] buf = new byte[1024];private int curChar,numChars;public FastReader() { this(System.in); }public FastReader(InputStream is) { mIs = is;}
        public int read() {if (numChars == -1) throw new InputMismatchException();if (curChar >= numChars) {curChar = 0;try { numChars = mIs.read(buf);} catch (IOException e) { throw new InputMismatchException();}if (numChars <= 0) return -1; }return buf[curChar++];}
        public String nextLine(){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 String next(){int c = read();while (isSpaceChar(c)) c = read();StringBuilder res = new StringBuilder();do {res.appendCodePoint(c);c = read();}while (!isSpaceChar(c));return res.toString();}
        public long l(){int c = read();while (isSpaceChar(c)) c = read();int sgn = 1;if (c == '-') { sgn = -1 ; c = read() ; }long res = 0; do{ if (c < '0' || c > '9') throw new InputMismatchException();res *= 10 ; res += c - '0' ; c = read();}while(!isSpaceChar(c));return res * sgn;}
        public int i(){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;}
        public double d() throws IOException {return Double.parseDouble(next()) ;}
        public boolean isSpaceChar(int c) { return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1; }
        public boolean isEndOfLine(int c) { return c == '\n' || c == '\r' || c == -1; }
        public void scanIntArr(int [] arr){ for(int li=0;li<arr.length;++li)arr[li]=i();}
        public void printIntArr(int [] arr){ for(int li=0;li<arr.length;++li)pw.print(arr[li]+" ");pw.println();}
        public void printLongArr(long [] arr){ for(int li=0;li<arr.length;++li)pw.print(arr[li]+" ");pw.println();}
        public void scanIntIndexArr(int [] arr){ for(int li=0;li<arr.length;++li){ arr[li]=i()-1;}}
        public void scanLongArr(long [] arr){for (int i=0;i<arr.length;++i){arr[i]=l();}}
        public void shuffle(int [] arr){ for(int i=arr.length;i>0;--i) { int r=(int)(Math.random()*i); int temp=arr[i-1]; arr[i-1]=arr[r]; arr[r]=temp; } }
        public void shuffle(long [] arr){ for(int i=arr.length;i>0;--i) { int r=(int)(Math.random()*i); long temp=arr[i-1]; arr[i-1]=arr[r]; arr[r]=temp; } }
        public int swapIntegers(int a,int b){return a;} //Call it like this: a=swapIntegers(b,b=a)
    }
    public static void main(String[] args) throws IOException {

        FastReader fr = new FastReader();
        PrintWriter pw = new PrintWriter(System.out);

     /*
inputCopy
2
outputCopy
2
inputCopy
3
outputCopy
4
inputCopy
4
outputCopy
6
  */
        //Press Ctrl+Win+Alt+L for reformatting indentation

        int t = 1;

        for (int ti = 0; ti < t; ++ti) {
            int n = fr.i();

            int cur=n/2;
            int ans=cur*(cur+1);
            if(n%2==1)ans+=(n+1)/2;
            pw.println(ans);
        }
        pw.flush();
        pw.close();
    }
}