CodeForces 630C：Lucky Numbers

                                              Lucky Numbers

Description

The numbers of all offices in the new building of the Tax Office of IT City will have lucky numbers.

Lucky number is a number that consists of digits 7 and 8 only. Find the maximum number of offices in the new building of the Tax Office given that a door-plate can hold a number not longer than n digits.

Input

The only line of input contains one integer n (1 ≤ n ≤ 55) — the maximum length of a number that a door-plate can hold.

Output

Output one integer — the maximum number of offices, than can have unique lucky numbers not longer than n digits.

Sample Input

Input

2

Output

6

题意：带有7或8的数字幸运数，求n位数中最多有多少个幸运数。

思路：2^1+2^2+...+2^n.

代码：

#include<stdio.h>#include<math.h>int main(){int n,i;long long a; while(scanf("%d",&n)!=EOF){a=0;for(i=1;i<=n;i++)a+=pow(2,i);printf("%lld\n",a);}return 0;}