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.io.IOException;
import java.io.InputStream;
import java.math.BigInteger;
import java.util.InputMismatchException;

public class Segments
{
    public static void main(String[] args)
    {
        FastReader in = new FastReader(System.in);
        int n = in.nextInt();
        int layers = 0;

        for (int segmentLength = 1; segmentLength <= n; segmentLength++)
        {
            layers += countLayers(segmentLength, n);
        }
        System.out.println(layers);
    }

    static int countLayers(int segmentLength, int n)
    {
        int numberOfSegments = n - segmentLength + 1;
        int startIndex = 0, layers = 0;

        while (true)
        {
            for (int i = startIndex; i + segmentLength <= n; i += segmentLength)
            {
                numberOfSegments--;
            }

            layers++;

            if (numberOfSegments == 0)
            {
                break;
            } else
            {
                startIndex++;
            }
        }
        return layers;
    }
}

class FastReader{
	private boolean finished = false;
 
	private InputStream stream;

	private byte[] buf = new byte[1024];
	private int curChar;
	private int numChars;
	private SpaceCharFilter filter;
 
	public FastReader(InputStream stream){
		this.stream = stream;
	}
 
	public int read(){
		if (numChars == -1){
			throw new InputMismatchException();
		}
		if (curChar >= numChars){
			curChar = 0;
			try{
				numChars = stream.read (buf);
			} catch (IOException e){
				throw new InputMismatchException ();
			}
			if (numChars <= 0){
				return -1;
			}
		}
		return buf[curChar++];
	}
 
	public int peek(){
		if (numChars == -1){
			return -1;
		}
		if (curChar >= numChars){
			curChar = 0;
			try{
				numChars = stream.read (buf);
			} catch (IOException e){
				return -1;
			}
			if (numChars <= 0){
				return -1;
			}
		}
		return buf[curChar];
	}
 
	public int nextInt(){
		int c = read ();
		while (isSpaceChar (c))
			c = read ();
		int sgn = 1;
		if (c == '-'){
			sgn = -1;
			c = read ();
		}
		int res = 0;
		do{
			if(c==','){
				c = read();
			}
			if (c < '0' || c > '9'){
				throw new InputMismatchException ();
			}
			res *= 10;
			res += c - '0';
			c = read ();
		} while (!isSpaceChar (c));
		return res * sgn;
	}
 
	public long nextLong(){
		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 String nextString(){
		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 boolean isSpaceChar(int c){
		if (filter != null){
			return filter.isSpaceChar (c);
		}
		return isWhitespace (c);
	}
 
	public static boolean isWhitespace(int c){
		return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
	}
 
	private String readLine0(){
		StringBuilder buf = new StringBuilder ();
		int c = read ();
		while (c != '\n' && c != -1){
			if (c != '\r'){
				buf.appendCodePoint (c);
			}
			c = read ();
		}
		return buf.toString ();
	}
 
	public String nextLine(){
		String s = readLine0 ();
		while (s.trim ().length () == 0)
			s = readLine0 ();
		return s;
	}
 
	public String nextLine(boolean ignoreEmptyLines){
		if (ignoreEmptyLines){
			return nextLine ();
		}else{
			return readLine0 ();
		}
	}
 
	public BigInteger nextBigInteger(){
		try{
			return new BigInteger (nextString ());
		} catch (NumberFormatException e){
			throw new InputMismatchException ();
		}
	}
 
	public char nextCharacter(){
		int c = read ();
		while (isSpaceChar (c))
			c = read ();
		return (char) c;
	}
 
	public double nextDouble(){
		int c = read ();
		while (isSpaceChar (c))
			c = read ();
		int sgn = 1;
		if (c == '-'){
			sgn = -1;
			c = read ();
		}
		double res = 0;
		while (!isSpaceChar (c) && c != '.'){
			if (c == 'e' || c == 'E'){
				return res * Math.pow (10, nextInt ());
			}
			if (c < '0' || c > '9'){
				throw new InputMismatchException ();
			}
			res *= 10;
			res += c - '0';
			c = read ();
		}
		if (c == '.'){
			c = read ();
			double m = 1;
			while (!isSpaceChar (c)){
				if (c == 'e' || c == 'E'){
					return res * Math.pow (10, nextInt ());
				}
				if (c < '0' || c > '9'){
					throw new InputMismatchException ();
				}
				m /= 10;
				res += (c - '0') * m;
				c = read ();
			}
		}
		return res * sgn;
	}
 
	public boolean isExhausted(){
		int value;
		while (isSpaceChar (value = peek ()) && value != -1)
			read ();
		return value == -1;
	}
 
	public String next(){
		return nextString ();
	}
 
	public SpaceCharFilter getFilter(){
		return filter;
	}
 
	public void setFilter(SpaceCharFilter filter){
		this.filter = filter;
	}
 
	public interface SpaceCharFilter{
		public boolean isSpaceChar(int ch);
	}
}
