【BZOJ 1426】收集邮票

题目传送门：http://www.lydsy.com/JudgeOnline/problem.php?id=1426

这货，我式子推得很奇怪QAQ
也不知道为什么是对的QAQ

#include<algorithm>
#include<iostream>
#include<cstring>
#include<cstdio>
#include<cstdio>
#include<cmath>
#define LL long long
#define abs(x) ((x)>0?(x):-(x))
using namespace std;

const int N = 10000+9;

int n;
double f[N],g[N];

inline int read(){
	char c=getchar(); int ret=0,f=1;
	while (c<'0'||c>'9') {if(c=='-')f=-1;c=getchar();}
	while (c<='9'&&c>='0') ret=ret*10+c-'0',c=getchar();
	return ret*f;
}

int main(){
	n = read(); f[n] = g[n] = 0;
	for (int i=n-1;~i;i--) f[i] = (double)n/(n-i) + f[i+1];
	for (int i=n-1;~i;i--) g[i] = f[i]*n/(n-i) + g[i+1];
	printf("%.2lf",g[0]);
	return 0;
}