import java.util.*;

class Solution{
    public static void main(String[] args){
        Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        if(n < 1){
            System.out.println("Invalid input");
            break;
        }
        int sum = 0;
        
        for(int i = 0; i < n; i++){
            if(i % 2 != 0){
                sum += i;
            }
        }
        
        System.out.println(sum);
    }
}