#P1057. C二级辅导-等差数列

C二级辅导-等差数列

<style type="text/css">p { margin-bottom: 0.21cm; }</style>

sum=2+5+8+11+14+,输入正整数n,求sum的前n项和。

Input

Output

Sample Input

2

Sample Output

7

HINT

Source