F - The Circumference of the Circle

#include <iostream>
#include<iomanip>
#include <cmath>
using namespace std;
#define PI 3.141592653589793
int main()
{
    double x1,y1,x2,y2,x3,y3;
    while(cin>>x1>>y1>>x2>>y2>>x3>>y3){
        double l,a1,b1,a2,b2,k1,k2,a,b;
        a1=x1/2+x2/2;
        a2=x1/2+x3/2;
        b1=y1/2+y2/2;
        b2=y1/2+y3/2;
        if(y1!=y2&&y3!=y1){
        k1=(x1-x2)/(y2-y1);
        k2=(x1-x3)/(y3-y1);
        a=(k1*a1-k2*a2+b2-b1)/(k1-k2);
        b=k1*(a-a1)+b1;
        }
        else if(y1==y2){
            k2=(x1-x3)/(y3-y1);
            a=(x1+x2)/2;
            b=k2*(a-a2)+b2;
        }
        else {
            k1=(x1-x2)/(y2-y1);
            a=(x1+x3)/2;
            b=k1*(a-a1)+b1;
        }
        l=2*PI*sqrt((a-x1)*(a-x1)+(b-y1)*(b-y1));
        cout.precision(2);
        cout.setf(ios::fixed); 
        cout<<l<<endl;
    }
    //system("pause");
    return 0;
}