71_分式相加

#include<iostream>
#include<stdio.h>
using namespace std;
int main()
{
    int N;
    cin>>N;
    double fz=2,fm=1;
    double d;
    double sum=0;
    while(N--){
        sum += fz/fm;
        d = fz;
        fz = fz + fm;
        fm = d;
    }
    printf("%.2lf",sum);
}