1117:计算圆的周长

#include<iostream>
#include<cmath>
#include <iomanip>
using namespace std;
int main()
{
   double pi=3.141592653589793;
double x1,y1,x2,y2,x3,y3,a,b,c;
while(cin>>x1>>y1>>x2>>y2>>x3>>y3)
{
   a=sqrt((x1-x2)*(x1-x2)+(y1-y2)*(y1-y2));
   b=sqrt((x3-x2)*(x3-x2)+(y3-y2)*(y3-y2));
   c=sqrt((x1-x3)*(x1-x3)+(y1-y3)*(y1-y3));
   double d=2*pi*a*b*c/(sqrt((a+b+c)*(a+b-c)*(b+c-a)*(a+c-b)));
   cout<<fixed<<setprecision(2)<<d<<endl;
}
return 0;
}
#include<iostream>
#include<cmath>
#include <io