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.*;
import java.util.*;

public class Solution{
    
    public static void main(String[] args) throws Exception{
        int N = in.readInt();
        int ans = 0;
        for(int i = 1; i <= N; i++){
            int layer = f(i,N);
            ans += layer;
            // System.out.println(i+"-> "+layer);
        }
        System.out.println(ans);
    }

    public static int f(int l, int N){
        int count = 0, max = N-l+1;
        // System.out.println("max="+max);
        for(int i = 0; i < l; i++){
            max -= (N-i)/(l);
            if(max >= 0)
                count++;
            if(max <= 0)
                break;
        }
        return count;
    }

    static Inputer in;
    static {
        in = new Inputer();
    }

    static class Inputer{
        BufferedReader br;
        Inputer(){
            try{
                br = new BufferedReader(new InputStreamReader(System.in));
            }
            catch(Exception e){}
        }
        public int readInt() throws Exception{
            return Integer.parseInt(readLine());
        }
        public long readLong() throws Exception{
            return Long.parseLong(readLine());
        }
        public int[] readA(String delim) throws Exception{
            String[] s = readLine().split(delim);
            int[] A = new int[s.length];
            for(int i = 0; i < s.length; i++)
                A[i] = Integer.parseInt(s[i]);
            return A;
        }
        public int[] readA() throws Exception{
            String[] s = readLine().split("\\s+");
            int[] A = new int[s.length];
            for(int i = 0; i < s.length; i++)
                A[i] = Integer.parseInt(s[i]);
            return A;
        }
        public long[] readLA() throws Exception{
            String[] s = readLine().split("\\s+");
            long[] A = new long[s.length];
            for(int i = 0; i < s.length; i++)
                A[i] = Long.parseLong(s[i]);
            return A;
        }
        public String readLine() throws Exception{
            return br.readLine();
        }
        public int[] copyA(int[] A){
            int[] B = new int[A.length];
            for(int i= 0 ; i < A.length; i++)
                B[i] = A[i];
            return B;
        }
    }
    static void shuffle(int[] A){
        int n = A.length;
        Random rand = new Random();
        for(int t = 0; t < A.length; t++){
            int i1 = rand.nextInt(n);
            int i2 = rand.nextInt(n);
            int tmp = A[i1];
            A[i1] = A[i2];
            A[i2] = tmp;
        }
    }
}