Language: C++

#include <iostream>
#include <algorithm>
#include <cstdio>
#include <cstring>
#include <cmath>
#include <vector>
using namespace std;
const double eps = 1e-8;
const double pi = acos(-1.0);
const double inf = 1e200;
inline int sgn(double x) {
    return x < -eps ? -1 : x > eps;
}
inline double sqr(double x) {
    return x * x;
}
inline double a2r(double a) { //隗貞コヲ蛻ー蠑ァ蠎ヲ
    return a * pi / 180;
}
inline double r2a(double r) { //蠑ァ蠎ヲ蛻ー隗貞コヲ
    return r / pi * 180;
}
struct P {
    double x,y;
    P(double x = 0,double y = 0):x(x), y(y) {};
    friend bool operator ==(const P& p1, const P& p2) {
        return sgn(p1.x - p2.x) == 0 && sgn(p1.y - p2.y) == 0;
    }
    friend bool operator <(const P& p1, const P& p2) {
        return sgn(p1.x - p2.x) == 0 ? sgn(p1.y - p2.y) < 0 : p1.x < p2.x;
    }
    friend bool operator >(const P& p1, const P& p2) {
        return sgn(p1.x - p2.x) == 0 ? sgn(p1.y - p2.y) > 0 : p1.x > p2.x;
    }
    friend P operator +(const P& p1, const P& p2) {
        return P(p1.x + p2.x, p1.y + p2.y);
    }
    friend P operator -(const P& p1, const P& p2) {
        return P(p1.x - p2.x, p1.y - p2.y);
    }
    friend P operator *(const P& p, double a) {
        return P(p.x * a,p.y * a);
    }
    friend P operator /(const P& p, double a) {
        return P(p.x / a,p.y / a);
    }
    friend inline double operator ^(const P& p1, const P& p2) { //轤ケ荵?        return p1.x * p2.x + p1.y * p2.y;
    }
    friend inline double operator *(const P& p1, const P& p2) { //蜿我ケ?        return p1.x * p2.y - p1.y * p2.x;
    }
    friend inline double dot(const P& o, const P& a, const P& b) {
        return (a - o) ^ (b - o);
    }
    friend inline double cross(const P& o, const P& a, const P& b) { //2蛟堺ク芽ァ貞ス「oab譛牙髄髱「遘ッ
        return (a - o) * (b - o);
    }
    friend double dist(const P& p1, const P& p2) {
        return sqrt((p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y));
    }
    friend double dist2(const P& p1, const P& p2) {
        return (p1.x - p2.x) * (p1.x - p2.x) + (p1.y - p2.y) * (p1.y - p2.y);
    }
    double len() const {
        return sqrt(x * x + y * y);
    }
    double len2() const {
        return x * x + y * y;
    }
    //豎ょ髄驥竣逧?栫隗抵シ瑚ァ貞コヲ闌?峩荳コ [0,2*pi)
    double angle() const {
        double t = atan2(y,x);
        return sgn(t) >= 0 ? t : t + 2 * pi;
    }
    P trunc(double l) const {
        l /= len();
        return P(x * l, y * l);
    }
    P turn_left() const { //豕募髄驥?        return P(-y, x);
    }
    P turn_right() const { //豕募髄驥?        return P(y, -x);
    }
    P rotate(double th, const P& p) const { //扈慕せp騾?慮髓郁スャth蠑ァ蠎ヲ
        double c = cos(th), s = sin(th);
        return P(c * (x - p.x) - s * (y - p.y) + p.x, s * (x - p.x) + c * (y - p.y) + p.y);
    }
    bool in() {
        return scanf("%lf %lf", &x, &y) == 2;
    }
    void out() const {
        printf("%lf %lf\n", x, y);
    }
};

int main()
{
	double d;
	P p,v;
	while(cin>>d>>p.x>>p.y>>v.x>>v.y)
	{
		double vx=v.x,vy=v.y,x=p.x,y=p.y;	 
		double b1=x*vy-y*vx;
		//cout<<(p*v)<<"  "<<b1<<endl;
		if(sgn(p*v-b1)!=0))while(1);
		if(sgn((p*v)))
		{
			puts("impossible");
			continue;
		} 
		if (sgn(vx*x)<0||sgn(vy*y)<0)
		 {
			 double dis=sqrt(x*x+y*y);
			 if (sgn(dis-d)>0) printf("impossible\n");
			 else printf("%.6lf\n",dis);
		 }
		 else
		 {
		     double dis=sqrt(x*x+y*y);
			 dis=2.0-dis;
			 if (sgn(dis-d)>0) printf("impossible\n");
			 else printf("%.6lf\n",dis);
		 }

	}
}

Compiler output:
a.cc: In function 'double operator^(const P&, const P&)':
a.cc:48:5: warning: no return statement in function returning non-void [-Wreturn-type]
   48 |     }
      |     ^
a.cc: In function 'double operator*(const P&, const P&)':
a.cc:50:5: warning: no return statement in function returning non-void [-Wreturn-type]
   50 |     }
      |     ^
a.cc: In member function 'P P::turn_left() const':
a.cc:79:5: warning: no return statement in function returning non-void [-Wreturn-type]
   79 |     }
      |     ^
a.cc: In member function 'P P::turn_right() const':
a.cc:81:5: warning: no return statement in function returning non-void [-Wreturn-type]
   81 |     }
      |     ^
a.cc: In function 'int main()':
a.cc:103:35: error: expected primary-expression before ')' token
  103 |                 if(sgn(p*v-b1)!=0))while(1);
      |                                   ^