1426: 收集邮票

期望乘法独立….

#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);
}