C++ :

#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
using namespace std;
inline double abs1(double a)
{
    return a>0?a:-a;
}
inline double get(double x1,double y1,double x2,double y2)
{
    return sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
}
int main()
{
    double x1,x2,y1,y2,x,y,a,b,c;
    while(1)
    {
        scanf("%lf%lf%lf%lf%lf%lf",&x1,&y1,&x2,&y2,&x,&y);
        if(x1==0 && x2==0 && y1==0 && y2==0 && x==0 && y==0)
            break;
        double A,B,C,ans;
        A=get(x2,y2,x,y);
        B=get(x1,y1,x,y);
        C=get(x1,y1,x2,y2);
        if((A*A)>=(B*B+C*C))
        {
            ans=B;
        }
        else if((B*B)>=(A*A+C*C))
        {
            ans=A;
        }
        else
        {
            a=y2-y1;
            b=x1-x2;
            c=x2*y1-x1*y2;
            double shang,xia;
            shang=abs1(a*x+b*y+c);
            double xia1=a*a+b*b;
            xia=sqrt(xia1);
            ans=shang/xia;
        }
        printf("%.2lf\n",abs1(ans));
    }
    return 0;
}