import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt();
        long ans;
        if (n % 2 == 0) {
            long base = (n - 2) / 2;
            ans = (1 + base) * base;
        } else {
            long base = (n - 2) / 2;
            ans = (1 + base) * base;
            ans += n / 2;
        }
        System.out.println(ans);
    }
}