help plz...




#include<cstdio>
#include<cmath>
#include<algorithm>
using namespace std;
#define rep(i,n) for(int i=0;i<n;i++)

struct Point{
  int x,y;
  Point(int x=0,int y=0):x(x),y(y){}
  void Read(){ scanf("%d %d",&x,&y);}
  int operator ^ (Point o){ return x*o.y-o.x*y;}
  int operator * (Point o){ return x*o.x+y*o.y;}
  Point operator - (Point o){ return Point(x-o.x,y-o.y);}
}P[10000],buf;

double dis(double a,double b,double c,Point P){
  return (a*(double)P.x+b*(double)P.y+c)/(sqrt(a*a+b*b));
}

double dis(Point p,Point q){
     return sqrt((double)((q-p)*(q-p)));
}

int n;
int main(){
  buf.Read(); scanf("%d",&n);
  rep(i,n) P[i].Read(); P[n]=P[0]; P[n+1]=P[1];

  bool inCon=1;
  rep(i,n) {
    if( ((P[i+1]-P[i]) ^ (buf-P[i])) * ((P[i+2]-P[i+1]) ^ (buf-P[i+1])) <0 ){
      inCon=0; break;
    }
  }
  if(inCon) {printf("0.000"); return 0;}


  double ans=19970815.0;
  rep(i,n){
    Point N(P[i+1].y-P[i].y,P[i].x-P[i+1].x); // fa xiang
    double c=-(N*P[i]);
    double d= ((P[i]-P[i+1])*(buf-P[i+1])) * ((P[i+1]-P[i])*(buf-P[i])) >0
     ?dis(N.x,N.y,c,buf)
     : min(dis(P[i],buf),dis(P[i+1],buf));

    if(ans>d) ans=d;
  }



  printf("%.03lf",ans*2);
}