foo.cc: In function 'double ojld(int, int)':
foo.cc:18:12: error: 'sqrt' was not declared in this scope
     return sqrt((p[i].x-p[j].x)*(p[i].x-p[j].x)
            ^~~~
foo.cc:18:12: note: suggested alternative: 'qsort'
     return sqrt((p[i].x-p[j].x)*(p[i].x-p[j].x)
            ^~~~
            qsort

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