foo.cc: In function 'int gcd(int, int)':
foo.cc:27:11: error: '__gcd' was not declared in this scope
     int p=__gcd(a,b);
           ^~~~~
foo.cc:27:11: note: suggested alternative:
In file included from /usr/include/c++/7/algorithm:62:0,
                 from foo.cc:14:
/usr/include/c++/7/bits/stl_algo.h:1232:5: note:   'std::__gcd'
     __gcd(_EuclideanRingElement __m, _EuclideanRingElement __n)
     ^~~~~
foo.cc:27:9: warning: unused variable 'p' [-Wunused-variable]
     int p=__gcd(a,b);
         ^
foo.cc: In function 'int main()':
foo.cc:39:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d%d",&x0,&y0);
     ~~~~~^~~~~~~~~~~~~~~~

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