Language: C++

#define EPS (1e-6)

using namespace std;

int main(){
	double d;
	
	while(1){
		double px,py,vx,vy;
		cin >> d;
		if(!(d)) break;
		cin >> px >> py >> vx >> vy;
		
		if(fabs(py * vx - px * vy) < EPS){
			if(vx * px < 0 || vy * py < 0){
				double x = sqrt(px*px + py*py);
				if( x < d + EPS) cout << x << endl;
				else cout << "impossible" << endl;
			}
			else if (vx * px > 0 || vy * py > 0){
				double x = 2.0 - sqrt(px*px + py*py);
				if ( x < d + EPS) cout << x << endl;
				else cout << "impossible" << endl;
			} else cout << "impossible" << endl;		
		} else cout << "impossible" << endl;
		
	}
	
	return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:10:17: error: 'cin' was not declared in this scope
   10 |                 cin >> d;
      |                 ^~~
a.cc:1:1: note: 'std::cin' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
  +++ |+#include <iostream>
    1 | #define EPS (1e-6)
a.cc:14:20: error: 'fabs' was not declared in this scope
   14 |                 if(fabs(py * vx - px * vy) < EPS){
      |                    ^~~~
a.cc:16:44: error: 'sqrt' was not declared in this scope
   16 |                                 double x = sqrt(px*px + py*py);
      |                                            ^~~~
a.cc:17:50: error: 'cout' was not declared in this scope
   17 |                                 if( x < d + EPS) cout << x << endl;
      |                                                  ^~~~
a.cc:17:50: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:17:63: error: 'endl' was not declared in this scope
   17 |                                 if( x < d + EPS) cout << x << endl;
      |                                                               ^~~~
a.cc:1:1: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
  +++ |+#include <ostream>
    1 | #define EPS (1e-6)
a.cc:18:38: error: 'cout' was not declared in this scope
   18 |                                 else cout << "impossible" << endl;
      |                                      ^~~~
a.cc:18:38: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:18:62: error: 'endl' was not declared in this scope
   18 |                                 else cout << "impossible" << endl;
      |                                                              ^~~~
a.cc:18:62: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
a.cc:21:50: error: 'sqrt' was not declared in this scope
   21 |                                 double x = 2.0 - sqrt(px*px + py*py);
      |                                                  ^~~~
a.cc:22:51: error: 'cout' was not declared in this scope
   22 |                                 if ( x < d + EPS) cout << x << endl;
      |                                                   ^~~~
a.cc:22:51: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:22:64: error: 'endl' was not declared in this scope
   22 |                                 if ( x < d + EPS) cout << x << endl;
      |                                                                ^~~~
a.cc:22:64: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
a.cc:23:38: error: 'cout' was not declared in this scope
   23 |                                 else cout << "impossible" << endl;
      |                                      ^~~~
a.cc:23:38: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:23:62: error: 'endl' was not declared in this scope
   23 |                                 else cout << "impossible" << endl;
      |                                                              ^~~~
a.cc:23:62: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
a.cc:24:32: error: 'cout' was not declared in this scope
   24 |                         } else cout << "impossible" << endl;
      |                                ^~~~
a.cc:24:32: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:24:56: error: 'endl' was not declared in this scope
   24 |                         } else cout << "impossible" << endl;
      |                                                        ^~~~
a.cc:24:56: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'
a.cc:25:24: error: 'cout' was not declared in this scope
   25 |                 } else cout << "impossible" << endl;
      |                        ^~~~
a.cc:25:24: note: 'std::cout' is defined in header '<iostream>'; this is probably fixable by adding '#include <iostream>'
a.cc:25:48: error: 'endl' was not declared in this scope
   25 |                 } else cout << "impossible" << endl;
      |                                                ^~~~
a.cc:25:48: note: 'std::endl' is defined in header '<ostream>'; this is probably fixable by adding '#include <ostream>'