题解 | #等差数列#

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;

public class Main {
    public static void main(String[] args) {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String a;
        try {
            a = in.readLine();
            in.close();
        } catch (IOException e) {
            e.printStackTrace();
            throw new RuntimeException(e);
        }
        int n = parse(a), i = 0, sum = 0, tmp = 0;
        while (i < n) {
            tmp = 3 * i;
            tmp += 2;
            sum += tmp;
            i++;
        }
        System.out.print(sum);
    }

    static int parse(String a) {
        int i = 0, l = a.length(), n = 0;
        char[] chrAy = new char[l];
        a.getChars(0, l, chrAy, 0);
        while (i < l) {
            if ((chrAy[i] - '0' | '9' - chrAy[i]) > 0) {
                n *= 10;
                n += chrAy[i] - '0';
            }
            i++;
        }
        return n;
    }
}