/in/foo.cc: In function 'int main()':
/in/foo.cc:14:97: warning: format '%lf' expects argument of type 'double', but argument 2 has type 'int' [-Wformat=]
   printf ("%.1lf\n", (abs ((x2 * y3 - x3 * y2) + (x3 * y1 - x1 * y3) + (x1 * y2 - x2 * y1))) / 2);
                                                                                                 ^