ural 1215 Exactness of Projectile Hit

 #include <cstdio>
 #include <cstring>
 #include <cmath>
 #include <algorithm>
 #define maxn 20000
 using namespace std;
 const double eps=1e-;

 inline double sqr(double x)
 {
     return x*x;
 }
 inline int dcmp(double x)
 {
     if(fabs(x)<eps) return ;
     else return x<?-:;
 }
 struct node
 {
     double x,y;
 }p[maxn];

 double dis(node a,node b)
 {
     return sqrt(sqr(a.x-b.x)+sqr(a.y-b.y));
 }

 double cross(node a,node b,node c)
 {
     return (c.x-a.x)*(b.y-a.y)-(c.y-a.y)*(b.x-a.x);
 }

 double dot(node a,node b)
 {
     return a.x*b.x+a.y*b.y;
 }
 bool in(node *p,int n,node a)
 {
     p[n]=p[];p[n+]=p[];
     for(int i=; i<n; i++)
     {
         if(-cross(p[i],p[i+],a)*cross(p[i+],p[i+],a)>eps)
         {
             return false;
         }
     }
     return true;
 }

 double disten(node pp,node a,node b)
 {
     if(a.x==b.x&&a.y==b.y) return dis(pp,a);
     node st1;st1.x=b.x-a.x; st1.y=b.y-a.y;
     node st2;st2.x=pp.x-a.x; st2.y=pp.y-a.y;
     node st3;st3.x=pp.x-b.x; st3.y=pp.y-b.y;
     if(dcmp(dot(st1,st2))<) return dis(pp,a);
     else if(dcmp(dot(st1,st3)>)) return dis(pp,b);
     else return fabs(cross(a,b,pp))/(dis(a,b));
 }

 int main()
 {
     int n;
     node c;
     scanf("%lf%lf%d",&c.x,&c.y,&n);
     double min1=1e10;
     for(int i=; i<n; i++)
     {
         scanf("%lf%lf",&p[i].x,&p[i].y);
     }
     if(in(p,n,c))
     {
         printf("%.3lf\n",0.0);
     }
     else
     {
         for(int i=; i<n; i++)
         {
             min1=min(min1,disten(c,p[i],p[i+]));
         }
         printf("%.3lf\n",*min1);
     }
     return ;
 }