import java.io.*;
public class Ex030203{
	static long method(int n){
		int tn=1;
				
		for (int i=2;i<=n;i++){
			tn=tn+i;

		}
		return tn;
	}
	public static void main(String[] args){
		int n=0;
		Reader ir = new InputStreamReader(System.in);
		BufferedReader r = new BufferedReader(ir);
		String sc="";
		System.out.print(" n = ");
		try{
			sc = r.readLine();
		}catch (IOException e){}
		try{
			n=Integer.parseInt(sc);
		}catch (NumberFormatException e){
			System.out.println("輸入的數(shù)據(jù)不是整數(shù)");
			System.exit(1);
		}
		System.out.println("t(n) = t(n-1) + n = "+method(n));
	}
}