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.util.*;
import java.io.*;
import java.math.*;
 
    public  class Sample implements Runnable
    { 
          
          
          public  static void solve()
          {   
            int n=i();
            int[] a=new int[n+1];
            for(int i=n;i>=1;i--)
            {
              a[i]=i;
            }
            int ans=0;
            for(int i=n;i>=1;i--)
            {
              if(a[i]<=0)break;
              ans+=a[i];
              for(int j=i-1;j>=1;j--)
              {
                if(a[j]>0)a[j]--;
              }
            }
            out.println(ans);

          }
          public static int upperbound(int[] a,int low,int high,int key)
          { 
             
            while(low<high)
            {
              int mid=(low+high)>>1;
              if(a[mid]>key)
              {
                high=mid;
              }
              else
              {
                low=mid+1;
              }
            }
            return low;
          }
          public static int lowerbound(int[] a,int low,int high,int key)
          { 
            while(low<high)
            {
              int mid=(low+high)>>1;
              if(a[mid]>=key)
              {
                high=mid;
              }
              else
              {
                low=mid+1;
              }
            }
            return low;
          }
           
 
          
          public void run()
          {   
              solve();
              out.close();
          }

          public static void main(String[] args) throws IOException
          { 
                new Thread(null, new Sample(), "whatever", 1<<26).start();
          }
         static class Pair implements Comparable<Pair>
         {
            long a;
            int b;
            
            Pair(){}
            Pair(long a,int b)
            {
                       this.a=a;
                       this.b=b;
            }
   
            public int compareTo(Pair x)
           {
                return Long.compare(x.a,this.a);
           }
        }
        
       
         
    ////////////////////////////////////////////////////// Merge Sort ////////////////////////////////////////////////////////////////////////
     
   static class Merge 
   {

        public static void sort(long inputArr[]) 
        {
            int length = inputArr.length;
            doMergeSort(inputArr,0, length - 1);
        }

        private static void doMergeSort(long[] arr,int lowerIndex, int higherIndex) 
        {        
            if (lowerIndex < higherIndex) {
                int middle = lowerIndex + (higherIndex - lowerIndex) / 2;
                doMergeSort(arr,lowerIndex, middle);
                doMergeSort(arr,middle + 1, higherIndex);
                mergeParts(arr,lowerIndex, middle, higherIndex);
            }
        }

        private static void mergeParts(long[]array,int lowerIndex, int middle, int higherIndex) 
        {
            long[] temp=new long[higherIndex-lowerIndex+1];
            for (int i = lowerIndex; i <= higherIndex; i++) 
            {
                temp[i-lowerIndex] = array[i];
            }
            int i = lowerIndex;
            int j = middle + 1;
            int k = lowerIndex;
            while (i <= middle && j <= higherIndex) 
            {
                if (temp[i-lowerIndex] < temp[j-lowerIndex]) 
                {
                    array[k] = temp[i-lowerIndex];
                    i++;
                } else {
                    array[k] = temp[j-lowerIndex];
                    j++;
                }
                k++;
            }
            while (i <= middle) 
            {
                array[k] = temp[i-lowerIndex];
                k++;
                i++;
            }
            while(j<=higherIndex)
            {
                array[k]=temp[j-lowerIndex];
                k++;
                j++;
            }
        }

    }
        
      
    /////////////////////////////////////////////////////////// Methods ////////////////////////////////////////////////////////////////////////


     static boolean isPal(String s)
     {
        for(int i=0, j=s.length()-1;i<=j;i++,j--)
        {
                if(s.charAt(i)!=s.charAt(j)) return false;
        }
        return true;
     }
     static String rev(String s)
     {
                StringBuilder sb=new StringBuilder(s);
                sb.reverse();
                return sb.toString();
     }
     static int gcd(int a,int b){return (a==0)?b:gcd(b%a,a);}
     static long gcdExtended(long a,long b,long[] x)
     {

        if(a==0){
            x[0]=0;
            x[1]=1;
            return b;
        }
        long[] y=new long[2];
        long gcd=gcdExtended(b%a, a, y);

        x[0]=y[1]-(b/a)*y[0];
        x[1]=y[0];

        return gcd;
    }

     boolean findSum(int set[], int n, long sum)
    {
      if (sum == 0)
         return true;
      if (n == 0 && sum != 0)
         return false;
      if (set[n-1] > sum)
         return findSum(set, n-1, sum);
      return findSum(set, n-1, sum) ||findSum(set, n-1, sum-set[n-1]);
    }
   
      public static long modPow(long base, long exp, long mod)
     {
        base = base % mod;
        long result = 1;
        while (exp > 0)
       {
            if (exp % 2 == 1) 
            {
                result = (result * base) % mod;
            }
            base = (base * base) % mod;
            exp = exp >> 1;
       }
        return result;
    }
 
    static long[] fac;
    static long[] inv;
    static long mod=(long)1e9+7;
    public static void cal() 
    {
        fac = new long[1000005];
        inv = new long[1000005];
        fac[0] = 1;
        inv[0] = 1;
        for (int i = 1; i <= 1000000; i++)
        {
            fac[i] = (fac[i - 1] * i) % mod;
            inv[i] = (inv[i - 1] * modPow(i, mod - 2, mod)) % mod;
        }
    }
 
    public static long ncr(int n, int r) 
    {
        return (((fac[n] * inv[r]) % mod) * inv[n - r]) % mod;
    }

 
////////////////////////////////////////// Input Reader ////////////////////////////////////////////////////////////////////////////////////////////////////
     
    static InputReader sc = new InputReader(System.in);
    static PrintWriter out= new PrintWriter(System.out);   
    
    static class InputReader {

          private final InputStream stream;
          private final byte[] buf = new byte[8192];
          private int curChar, snumChars;
          private SpaceCharFilter filter;

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

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

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

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

          public int[] nextIntArray(int n)
          {
                  int a[] = new int[n];
                  for (int i = 0; i < n; i++) 
                  {
                          a[i] = nextInt();
                  }
                  return a;
          }

          public long[] nextLongArray(int n)
          {
                  long a[] = new long[n];
                  for (int i = 0; i < n; i++) 
                  {
                          a[i] = nextLong();
                  }
                  return a;
          }

          
          public String nextLine()
         {
                  int c = snext();
                  while (isSpaceChar(c))
                          c = snext();
                  StringBuilder res = new StringBuilder();
                  do {
                          res.appendCodePoint(c);
                          c = snext();
                  } while (!isEndOfLine(c));
                  return res.toString();
          }

          public boolean isSpaceChar(int c) 
          {
                  if (filter != null)
                          return filter.isSpaceChar(c);
                  return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
          }

          private boolean isEndOfLine(int c) 
          {
                  return c == '\n' || c == '\r' || c == -1;
          }

          public interface SpaceCharFilter 
          {
                  public boolean isSpaceChar(int ch);
          }

    }

    static int i()
    {
        return sc.nextInt();
    }
    static long l(){
        return sc.nextLong();
    }
    static int[] iarr(int n)
    {
        return sc.nextIntArray(n);
    }
    static long[] larr(int n)
    {
        return sc.nextLongArray(n);
    }
    static String s(){
        return sc.nextLine();
    }
  
 
 } 