Problem:
Score : 100 points

Problem Statement

There are N children in AtCoder Kindergarten. Mr. Evi will arrange the children in a line, then give 1 candy to the first child in the line, 2 candies to the second child, ..., N candies to the N-th child. How many candies will be necessary in total?

Constraints

1≦N≦100

Input

The input is given from Standard Input in the following format:

N

Output

Print the necessary number of candies in total.

Sample Input 1

3

Sample Output 1

6

The answer is 1+2+3=6.

Sample Input 2

10

Sample Output 2

55

The sum of the integers from 1 to 10 is 55.

Sample Input 3

1

Sample Output 3

1

Only one child. The answer is 1 in this case.

Solution in Rust (1.15.1) (judge verdict: Wrong Answer)

fn main() {
    let s: String = {
        let mut buffer = String::new();
        std::io::stdin().read_line(&mut buffer).unwrap();

        buffer.trim().to_string()
    };

    let mut unbalanced = None;
    let mut mae = (None, None);

    for (i, c) in s.chars().enumerate() {
        if let Some(mc) = mae.0 {
            if mc == c {
                unbalanced = Some((i - 1, i));
                break;
            }
        }

        if let Some(mc) = mae.1 {
            if mc == c {
                unbalanced = Some((i - 2, i));
                break;
            }
        }

        mae.1 = mae.0;
        mae.0 = Some(c);
    }

    if let Some((i, j)) = unbalanced {
        println!("{} {}", i + 1, j + 1);
    } else {
        println!("-1 -1");
    }
}