UVA-10242 :: Fourth Point !!

#include <stdio.h>
int main ()
{
     double x1 ,x2 ,x3 ,x4 ,y1 ,y2 ,y3 ,y4 , h ,k ,m,n, x, y ,p,a,b,q ;

     while (scanf ("%lf %lf %lf%lf %lf %lf %lf %lf",&x1 ,&y1 ,&x2, &y2 ,&x3,&y3 ,&x4 ,&y4) == 8 )
     {
          if (x1==x2 && y1==y2)  {p = x3 ; q = x4 ; a = y3; b =y4 ; m =x1 ;n =y1 ;}
          else if (x1==x3 && y1==y3) {p = x2 ; q = x4 ; a = y2; b =y4 ;m=x1 ; n=y1; }
          else if (x1==x4 && y1==y4) {p = x3 ; q = x2 ; a = y2; b =y3 ;m=x1 ; n=y1;}
          else if (x2==x3 && y2==y3) {p = x1 ; q = x4 ; a = y1; b =y4 ; m=x2 ;n=y2;}
          else if (x2==x4 && y2==y4) {p = x3 ; q = x1 ; a = y3; b =y1 ; m=x2; n=y2;}
          else if (x3==x4 && y3==y4) {p = x2 ; q = x1 ; a = y2; b =y1 ; m=x3; n=y3;}

          h = (p + q) /2.00 ;
          k = (a + b) /2.00 ;

          x = 2.00*h -m ;
          y = 2.00*k - n ;

          printf ("%.3lf %.3lf\n",x ,y) ;

     }

return 0 ;
}