luogu_1984 [SDOI2008]烧水问题

#include <cstdio>
using namespace std;
double ans; 
int n;

int main(){
    scanf("%d",&n);
    double ret=420000.00/n;
    for(register double i=1;i<=n;i++){
        ans+=ret;
        ret-=ret*0.5/i;
    }
    printf("%.2lf\n",ans);
    return 0;
}