题解 | #等差数列#

public class Main {
     public static void main(String[] args){
         Scanner sc = new Scanner(System.in);
         if(sc.hasNext()){
             int num = sc.nextInt();
             int sum = 0;
             for(int i =0;i<num;i++){
                 sum +=count(i);
             }
             System.out.print(sum);
         }   
     }
    public static int count(int i){
        if(i==0){
            return 2;
        }
        return count(i-1)+3;
    }
}