foo.cc: In function 'bool pd(std::__cxx11::string, std::__cxx11::string)':
foo.cc:4:59: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
 #define rep(i,x,y) if ((x)<=(y)) for (register int i=(x);i<=(y);i++)
                                                           ^
foo.cc:8:2: note: in expansion of macro 'rep'
  rep(i,0,s1.length()-1)
  ^~~

[Hydro](https://hydro.ac)提供评测服务