The root of a cubic equation

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

#define int long long
#define double long double

int a , b , c , d;

double f(double x , int a , int b , int c , int d){
    double s = a * x * x * x + b * x * x + c * x + d;
    return s;
}

signed main()
{
    cout.precision(6);
    cin >> a >> b >> c >> d;
    double r = 1.0;
    while(true){
        if((f(r , a , b , c , d) * f(-r , a , b , c , d)) < 0){
            break;
        }else{
            r *= 2;
        }
    }
    double l = -r;
    while((l + 1e-6) <= r){
        double m = (l + r) * 1.0 / 2;
        if((f(m , a , b , c , d) * f(-m , a , b , c , d)) < 0){
            r = m;
        }else{
            l = m;
        }
    }
    double x = abs(f(l , a , b , c , d)) , y = abs(f(-l , a , b , c , d));
    if(x > y)
        l = -l;
    cout << fixed << l << endl;
}