Here is my C++ source...



#include <stdio.h>
#include <math.h>

struct SPoint
{
    double x,y;
}a[101];

#define sqr(a) ((a)*(a))
#define min(a,b) ((a>b) ? b : a)

//Distance between point x and point y
double l(SPoint x, SPoint y)
{
    return sqrt(sqr(x.x-y.x)+sqr(x.y-y.y));
}

//Squaer of the distance between point x and point y
double l2(SPoint x, SPoint y)
{
    return sqr(x.x-y.x)+sqr(x.y-y.y);
}

//cos(<xyz)
double cos(SPoint x, SPoint y, SPoint z)
{
    return (l2(x,y)+l2(z,y)-l2(x,z))/(2*l(x,y)*l(z,y));
}

//face of triangle with points x,y,z
double face(SPoint x, SPoint y, SPoint z)
{
    return fabs((x.x - y.x)*(y.y - z.y)+(z.x - y.x)*(x.y -
y.y))/2;
}


double h(SPoint x, SPoint y, SPoint z)
{
    return (2*face(x,y,z))/l(x,z);
}


int n, i;
double rez, face_all, face_point;

void main()
{
    scanf("%lf%lf%d",&a[0].x, &a[0].y, &n);
    scanf("%lf%lf",&a[1].x, &a[1].y);
    rez = l(a[0],a[1]);
    face_all = 0.00000;
    face_point = 0.00000;
    for(i = 2; i <= n; i++)
    {
        scanf("%lf%lf",&a[i].x, &a[i].y);
        if(i > 2) face_all += face(a[1], a[i-1], a[i]);
        face_point += face(a[0],a[i-1],a[i]);
        if(cos(a[i-1],a[0],a[i]) > 0.0000000 && cos(a[i],a
[0],a[i-1]) > 0.0000000)
        {
            rez = min(rez, l(a[0], a[i-1]));
            rez = min(rez, l(a[0], a[i]));
        }
        else rez = min(rez, h(a[i-1],a[0],a[i]));
    }
    if(cos(a[1],a[0],a[n]) > 0.000000 && cos(a[n],a[0],a[1]) >
0.0000000)
    {
        rez = min(rez, l(a[0], a[1]));
        rez = min(rez, l(a[0], a[n]));
    }
    else rez = min(rez, h(a[1],a[0],a[n]));
    face_point += face(a[0],a[n],a[1]);
    if(fabs(face_all-face_point)<0.000000001) rez = 0.0000;
    printf("%.3lf\n",2*rez);
}