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>
//pakage joney_000[let_me_start]
//
import java.util.*;
import java.lang.*;
import java.io.*;
import java.math.*;
/*
 * Author    : joney_000[let_me_start]
 * Algorithm : N/A
 * Platform  : N/A
 *
 */


/*    The Main Class                */
public class A{

  private InputStream inputStream ;
  private OutputStream outputStream ;
  private FastReader in ;
  private PrintWriter out ;
  /*
    Overhead [Additional Temporary Strorage] but provides memory reusibility for multiple test cases.
    Size Limit : 10^5 + 4 
  */
  private final int BUFFER = 50000;
  private int    tempints[] = new int[BUFFER];
  private long   templongs[] = new long[BUFFER];
  private double tempdoubles[] = new double[BUFFER];
  private char   tempchars[] = new char[BUFFER];
  //private final long mod = 1000000000+7;
  private final int  INF  = Integer.MAX_VALUE / 10;
  private final long INF_L  = Long.MAX_VALUE / 10;

  public A(){}
  public A(boolean stdIO)throws FileNotFoundException{
    // stdIO = false;
    if(stdIO){
      inputStream = System.in;
      outputStream = System.out;
    }else{
      inputStream = new FileInputStream("input.txt");
      outputStream = new FileOutputStream("output.txt");
    }
    in = new FastReader(inputStream);
    out = new PrintWriter(outputStream);
  }

  void run()throws Exception{
    // int tests = i();
    // for(int t = 1; t <= tests; t++){
    //   clear();
      int n = i();
      int cnt[] = new int[n + 5];

      for(int i = 1; i <= n + 1; i++){
        for(int j = i + 1; j <= n + 1; j++){
          cnt[i] += 1;
          cnt[j] -= 1;
        }
      }
      long ans = 0;
      long sum = 0;
      for(int i = 1; i <= n + 1; i++){
        sum += cnt[i];
        ans = Math.max(ans, sum);
      }
      out.write(""+ans+"\n");
    // }
  }// end run

  void clear(){
    
  }

  //****************************** Gerenal Utilities ***********************//

  void print_r(Object... o){
    out.write("\n"+Arrays.deepToString(o)+"\n");
    out.flush();
  }

  boolean isPrime(long n){
    if(n == 1)return false;
    if(n <= 3)return true;
    if(n % 2 == 0)return false;
    for(int i = 2 ;i <= Math.sqrt(n); i++){
      if(n % i == 0)return false;
    }
    return true;
  }
  // sieve
  int[] primes(int n){       // for(int i=1;i<=arr.length-1;i++)out.write(""+arr[i]+" ");
    boolean arr[] = new boolean[n+1];
    Arrays.fill(arr,true);
    arr[1]=false;
    for(int i=2;i<=Math.sqrt(n);i++){
      if(!arr[i])continue;
      for(int j = 2*i ;j<=n;j+=i){
        arr[j]=false;
      }
    }
    LinkedList<Integer> ll = new LinkedList<Integer>();
    for(int i=1;i<=n;i++){  
      if(arr[i])ll.add(i);
    }
    n = ll.size();
    
    int primes[] = new int[n+1];
    for(int i=1;i<=n;i++){
      primes[i]=ll.removeFirst();
    }
    return primes;
  }

  long gcd(long a , long b){
    if(b==0)return a;
    return gcd(b , a%b);
  }

  long lcm(long a , long b){
    if(a==0||b==0)return 0;
    return (a*b)/gcd(a,b);
  }

  long mulmod(long a , long b ,long mod){
    if(a==0||b==0)return 0;
    if(b==1)return a;
    long ans = mulmod(a,b/2,mod);
    ans = (ans*2)% mod;
    if(b%2==1)ans = (a + ans)% mod;
    return ans;
  }
  
  long pow(long a , long b ,long mod){
    if(b==0)return 1;
    if(b==1)return a;
    long ans = pow(a,b/2,mod);
    ans = (ans * ans);
    if(ans >= mod )ans %= mod;

    if(b%2==1)ans = (a * ans);
    if(ans >= mod )ans %= mod;

    return ans;
  }
  // 20*20   nCr Pascal Table
  long[][] ncrTable(){
    long ncr[][] = new long[21][21];
    for(int i=0 ;i<=20 ;i++){ncr[i][0]=1;ncr[i][i]=1;}
      for(int j=0;j<=20 ;j++){
        for(int i=j+1;i<= 20 ;i++){
          ncr[i][j] = ncr[i-1][j]+ncr[i-1][j-1];
        }
      }
    return ncr;
  }
  //*******************************I/O******************************//  
  int i()throws Exception{
    //return Integer.parseInt(br.readLine().trim());
    return in.nextInt();
  }
  int[] is(int n)throws Exception{
  //int arr[] = new int[n+1];
    for(int i=1 ; i <= n ;i++)tempints[i] = in.nextInt();  
      return tempints;
  }
  long l()throws Exception{
    return in.nextLong();
  }
  long[] ls(int n)throws Exception{
    for(int i=1 ; i <= n ;i++)templongs[i] = in.nextLong();  
      return templongs;
  }

  double d()throws Exception{
    return in.nextDouble();
  }
  double[] ds(int n)throws Exception{
    for(int i=1 ; i <= n ;i++)tempdoubles[i] = in.nextDouble();  
      return tempdoubles;
  }
  char c()throws Exception{
    return in.nextCharacter();
  }
  char[] cs(int n)throws Exception{
    for(int i=1 ; i <= n ;i++)tempchars[i] = in.nextCharacter();  
      return tempchars;
  }
  String s()throws Exception{
    return in.nextLine();
  }
  BigInteger bi()throws Exception{
    return in.nextBigInteger();
  }
//***********************I/O ENDS ***********************//
//*********************** 0.3%f [precision]***********************//
/* roundoff upto 2 digits 
   double roundOff = Math.round(a * 100.0) / 100.0;
                    or
   System.out.printf("%.2f", val);
          
*/
/*
  print upto 2 digits after decimal
  val = ((long)(val * 100.0))/100.0;
  
*/    

    private void closeResources(){
     out.flush();
     out.close();
     return;
    }
    public static void main(String[] args) throws java.lang.Exception{
    //let_me_start Shinch Returns 


    /*  
        // Old Reader Writer
        BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
        BufferedWriter out=new BufferedWriter(new OutputStreamWriter(System.out));
    BufferedReader br=new BufferedReader(new FileReader("input.txt"));
        BufferedWriter out=new BufferedWriter(new FileWriter("output.txt"));
    */   
        A driver = new A(true);
        
        driver.run();

        driver.closeResources();
        return ;

      }  

    }

    class FastReader{

      private boolean finished = false;

      private InputStream stream;
      private byte[] buf = new byte[4 * 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);
      }
    }
    /******************** Pair class ***********************/

  class Pair implements Comparable<Pair>{
     public int a;
     public int b;
          public int c;
     public Pair(){
      this.a = 0;
      this.b = 0;
    }
    public Pair(int a,int b, int c){
      this.a = a;
      this.b = b;
      this.c = c;
    }
    public int compareTo(Pair p){
      if(this.a==p.a){
       return this.b-p.b;  
     }
     return p.a-this.a; 
   }
   public String toString(){
    return "a="+this.a+" b="+this.b;
  }

} 
