foo.cc: In function 'int main()':
foo.cc:12:18: warning: variable 'Left' set but not used [-Wunused-but-set-variable]
  register double Left,Right;
                  ^~~~
foo.cc:12:23: warning: variable 'Right' set but not used [-Wunused-but-set-variable]
  register double Left,Right;
                       ^~~~~
foo.cc:11:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%lf%lf%lf%lf",&A,&B,&C,&D);
  ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

自豪的采用HydroJudge(https://github.com/hydro-dev/Hydro)进行评测。