Petya Slonikov
During another practical session in parallel C, I, Andriy Serhiyovych, found myself with some free time. I decided to visit the computer lab on the second floor to check on the students' progress. On my way, I encountered some stairs and, with no one around, I thought it would be fun to see how many steps I could jump.
I became so absorbed in this activity that I didn't realize the students had already left. Then, a curious boy named Petya Slonikov approached me. After attending two hours of lectures and one hour of practical work on dynamic programming, he was eager to know one thing: how many different steps could I land on after making at least one jump?
Petya watched me for a while and concluded that I could jump no more than k steps at a time, and I never repeated the same number of steps in a jump because I dislike repetition. Just as he was about to suggest a solution, he remembered he urgently needed to submit 20 mandatory problems for credit and hurried off to complete them.
I found Petya's question intriguing and decided to present it as a problem in the olympiad.
Input
The first line contains a single integer k (1 ≤ k ≤ 1000).
Output
Output a single integer - the answer to Petya Slonikov's question.