#include <iostream>
#include <iomanip>
#include <gsl/gsl_sf_bessel.h>
using namespace std;
int main(void) {
  double x = 5.0;
  double y = gsl_sf_bessel_J0(x);
  cout<<setprecision(18);
  std::cout<<"J0("<<x<<") = "<<fixed<<y<<endl;
  return 0;
}
//output J0(5) = -0.177596771314338264