from functools import cache
import sys
num = int(input())
@cache
def dfs(n):
    if n == 1:
        return 2
    return dfs(n-1) + 3
res = 0
for i in range(1,num+1):
    res += dfs(i)
print(res)

斐波那契,但是这个应该是能总结出数学公式的,忘记怎么推导了