#include<iostream>
#include<stdio.h>
#include<math.h>
using namespace std;
struct point { double x;
               double y;
               double z; };
double pi=3.14159265358979323846;
int main () {
    point a,b,c;
    double tanga,tangb,distac,distbc,ang,arc,r,angpt;
    cin >> a.x >> a.y >> a.z >> b.x >> b.y >> b.z >> c.x >> c.y >> c.z >> r ;
    distac=sqrt((a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y)+(a.z-c.z)*(a.z-c.z));
    distbc=sqrt((b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y)+(b.z-c.z)*(b.z-c.z));
    tanga=sqrt(distac*distac-r*r);
    tangb=sqrt(distbc*distbc-r*r);
    angpt=acos(((a.x-c.x)*(b.x-c.x)+(a.y-c.y)*(b.y-c.y)+(a.z-c.z)*(b.z-c.z))/
                (sqrt((a.x-c.x)*(a.x-c.x)+(a.y-c.y)*(a.y-c.y)+(a.z-c.z)*(a.z-c.z))*
                 sqrt((b.x-c.x)*(b.x-c.x)+(b.y-c.y)*(b.y-c.y)+(b.z-c.z)*(b.z-c.z))))*(180/pi); //angle between the two vectors AC and BC
    ang=angpt-(((atan2(tanga,distac)+atan2(tangb,distbc))*(180/pi)));
    ang/=360;
    arc=ang*2*pi*r;
    printf("%.2lf",tanga+tangb+arc);
    cout << endl ;
    return 0;
}

I know that I have to check if there could be a straight line but I think that the sample test should be right for my program.
My program writes 19.78 not 19.71. Thanks in advance.