foo.cc: In function 'int main()':
foo.cc:9:4: error: 'sqrt' was not declared in this scope
  s=sqrt(p*(p-a)*(p-b)*(p-c));
    ^~~~
foo.cc:9:4: note: suggested alternative: 'qsort'
  s=sqrt(p*(p-a)*(p-b)*(p-c));
    ^~~~
    qsort

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