1426: 收集邮票

#include<bits/stdc++.h>
#define rep2(i,k,n) for(int i=k;i>=n;i--)
using namespace std;
typedef double db;
db g,f;int n;
int main(){
scanf("%d",&n);g=f=0.0;
rep2(i,n-1,0){
g+=n*1.0/(n-i)*f;
db kk=i*1.0/n;
g+=1.0/(1.0-kk)/(1.0-kk);
f+=n*1.0/(n-i);
}printf("%.2lf\n",g);
}