Buttons.java

/**
 * https://codeforces.com/problemset/problem/268/B tag: #implementation #math n, calculate
 * worse-case from n->1 n = 3; res = 3 + (2 + (2-1)*1) + (1 + (1-1)*2) n = 4; res = 4 + (3 +
 * (3-1)*1) + (2 + (2-1)*2) + (1 + (1-1)*3)
 */
import java.util.Scanner;

public class Buttons {

  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int n = sc.nextInt();
    int result = 0;
    for (int i = n; i > 0; i--) {
      result += i + (n - i) * (i - 1);
    }
    System.out.println(result);
  }
}

 /**
  * Better way (Mathematic) Tim so lan sai, trong 4 so: n = 4, toi da sai 3 lan, so lan bam: 1. n =
  * 3, toi da sai 2 lan, so lan bam: 2. n = 2, toi da sai 1 lan, so lan bam: 3. n = 1, toi da sai 0
  * lan, so lan bam: 4. .....
  *
  * <p>(n - i) * i n * i - i^2
  *
  * <p>k = k(k+1)(2k+1)/6
  *
  * <p>n(1 + 2 + .. n-1) [3 * n * n *(n-1) - (n-1) * n * (2n - 1)] / 6 [n * (n - 1) * (3n - (2n -
  * 1))] / 6
  *
  * <p>n * (n - 1) * (n + 1) / 6
  */