import java.util.*;
import java.lang.*;
import java.io.*;

// The main method must be in a class named "Main".
class Main {
    public static int sum_odd(int n ){
        int sum = 0 ; 
        for(int i=1;i<=n;i+=2){
            sum = sum + i ; 
            
        }
        return sum ;
    }
    
    
    public static void main(String[] args) {
        System.out.println(sum_odd(10) );
    }
}