求大佬看看哪里错了(5分)

#include<bits/stdc++.h>
using namespace std;
int main(){
    double a[31],n,sum=2.0/1;
    cin>>n;
    if(n==1)
    cout<<fixed<<setprecision(4)<<sum;
    else{
        a[0]=1;a[1]=2;
    for(int i=2;i<=n-1;i++){
        a[i]=a[i-1]+a[i-2];
        sum+=a[i]/a[i-1];
    }
   cout<<fixed<<setprecision(4)<<sum;}
    return 0;
    
    
    
    
}