import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n=sc.nextInt();
		int count=0;
		int sum=0;
		for(int k=1;true;k++) {
			// k 来控制 每次加的天数
		for(int p=0;p<k;p++) {
			sum=sum+k;
			count++;
			if(count==n) {
				System.out.print(sum);
				
				break;
			}	
			
			
			
			
		}
					
		
		if(count==n) {
			
			break;
		}			
			
			
		}
	}

}
/**************************************************************
	Problem: 1520
	User: admin
	Language: Java
	Result: Accepted
	Time:4247 ms
	Memory:40420 kb
****************************************************************/