poj 2242 The Circumference of the Circle

#include <iostream>
#include <cmath>
#include <iomanip>
using namespace std;
const double pi=3.141592653589793;
int main()
{
    double x1,y1,x2,y2,x3,y3,a,b,c,st,cir,r,p;
    while(cin>>x1>>y1>>x2>>y2>>x3>>y3)
    {
        a=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
        b=sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
        c=sqrt((x3-x2)*(x3-x2)+(y3-y2)*(y3-y2));

        p=(a+b+c)/2;

        st=sqrt(p*(p-a)*(p-b)*(p-c));

        r=a*b*c/(4*st);

        cir=2*pi*r;

        cout.setf(ios::fixed);
        cout.precision(2);
        cout<<cir<<endl;
    }
    return 0;
}