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.BufferedOutputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;

/**
 * <pre>
 * I'll tell you a riddle.
 * You're waiting for a train,
 * a train that will take you far away.
 * You know where you hope this train will take you,
 * but you don't know for sure. But it doesn't matter.
 * How can it not matter to you where that train will take you?
 * </pre>
 * @author bodmas
 */
public class ProblemB {

    private int solve(int n) {
        int sum = 0;
        while (n > 0) {
            int floor = n - 1 >> 1;
            floor *= 2;
            floor += 1;
            if (n % 2 == 0)
                floor += 1;
            n -= 2;
            sum += floor;
        }
        return sum;
    }

    public static void main(String[] args) throws IOException {
        try (BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
                PrintWriter out = new PrintWriter(new BufferedOutputStream(System.out))) {
            int n = Integer.parseInt(in.readLine());
            out.println(new ProblemB().solve(n));
        }
    }
}
