我这个蒟蒻莫名其妙只有5分。求调！！

#include <bits/stdc++.h>
using namespace std;

int main(){
    double c=0;
    int n;
    double a1=2,a2=0,a3=0,b1=1,b2=0,b3=0;
    cin>>n;
    c+=a1/b1;
    a3=a2;
    a2=a3+a1;
    a1=a3;
    b3=b2;
    b2=b1+b3;
    b1=b3;
    while(--n){
        c+=a2/b2;
        a3=a2;
        a2=a3+a1;
        a1=a3;
        b3=b2;
        b2=b1+b3;
        b1=b3;
    }
    printf("%.4lf",c);
    return 0;
}