4378: 求1+4+7+10+……+n之和

Memory Limit:128 MB Time Limit:1.000 S
Judge Style:Text Compare Creator:
Submit:80 Solved:36

Description

求1+4+7+10+……+n的等差数列之和,公差为3。( 1<=n<=1000)

Input

一个整数n( 1<=n<=1000)

Output

等差数列的和

Sample Input Copy

100

Sample Output Copy

1717