样例输出0.500000？

#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N=1e5+7;
const int INF=0x3f3f3f3f;

int T;
int n;double l,r;
ll a[N];

double f(double x){
    double ret=0;
    for(int i=n+1;i>0;i--) ret=ret*x+a[i];
    return ret;
}

int main()
{
	T=1;
	while(T--)
	{
		cin>>n>>l>>r;
		for(int i=1;i<=n+1;i++)
		{
			cin>>a[i];
		}
		double m1,m2;
		while(abs(r-l)>1e-9)
		{
			m1=l+(r-l)/3;
			m2=r-(r-l)/3;
			if(f(m1)<f(m2))
			{
				r=m2;
			}
			else
			{
				l=m1;
			}
		}
		printf("%.5f\n",(l+r)/2);
	}
	return 0;
}