CodeForces 630c

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

代码：

#include<stdio.h>
#include<math.h>
int main()
{
	int n;
	long long i,sum[100];
	sum[1]=2;
	for( i=2;i<=55;i++)
	{
		sum[i]=sum[i-1]+pow(2,i);
	}
	while(scanf("%d",&n)!=EOF)
	{
		printf("%lld\n",sum[n]);
	}
	return 0;
}

思路：找规律的题，开始做的时候晕了，竟然把longlong 对应的  lld  记成了ld！

规律是多一位数字（这个数字可以是7或8），所以就多了2*（上一位数）项，再加上原来的：sum[i]=sum[i-1]+pow(2,i);