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.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.util.TreeSet;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author Pradyumn
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        FastReader in = new FastReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskB {
        static TreeSet<TaskB.Pair> set = new TreeSet<>();
        public int count = 0;
        static Debug debug;

        public void solve(int testNumber, FastReader in, PrintWriter out) {
            debug = new Debug(out);
            int n = in.nextInt();
            for (int i = 0; i < n; ++i) {
                for (int j = i + 1; j <= n; ++j)
                    set.add(new TaskB.Pair(i, j));
            }
            while (!set.isEmpty()) {
                ++count;
                go(0, n);
            }
            out.println(count);
        }

        public static void go(int start, int end) {
            if (start == end) return;
            //debug.tr(count, start, end);
            for (int i = start; i < end; ++i) {
                for (int j = end; j > i; --j) {
                    if (set.contains(new TaskB.Pair(i, j))) {
                        set.remove(new TaskB.Pair(i, j));
                        //debug.tr("1", count, start, i);
                        if (start != i) go(start, i);
                        if (j != end) go(j, end);
                        return;
                    }
                }
            }
        }

        static class Pair implements Comparable<TaskB.Pair> {
            int x;
            int y;

            public Pair(int x, int y) {
                this.x = x;
                this.y = y;
            }


            public boolean equals(Object o) {
                if (o == this) return true;
                if (o.getClass() != this.getClass()) return false;

                TaskB.Pair p = (TaskB.Pair) o;
                return p.x == x && p.y == y;
            }


            public int compareTo(TaskB.Pair p) {
                if (p.x != x)
                    return x - p.x;
                return y - p.y;
            }

        }

    }

    static class FastReader {
        private InputStream stream;
        private byte[] buf = new byte[8192];
        private int curChar;
        private int pnumChars;

        public FastReader(InputStream stream) {
            this.stream = stream;
        }

        private int pread() {
            if (pnumChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= pnumChars) {
                curChar = 0;
                try {
                    pnumChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (pnumChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int nextInt() {
            int c = pread();
            while (isSpaceChar(c))
                c = pread();
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = pread();
            }
            int res = 0;
            do {
                if (c == ',') {
                    c = pread();
                }
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = pread();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        private boolean isSpaceChar(int c) {
            return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
        }

    }

    static class Debug {
        PrintWriter out;
        boolean oj;
        long timeBegin;
        Runtime runtime;

        public Debug(PrintWriter out) {
            oj = System.getProperty("ONLINE_JUDGE") != null;
            this.out = out;
            this.timeBegin = System.currentTimeMillis();
            this.runtime = Runtime.getRuntime();
        }

    }
}

