foo.cc: In function 'int main()':
foo.cc:21:6: warning: variable 'a' set but not used [-Wunused-but-set-variable]
  int a,b; a = read(); b = read();
      ^
foo.cc:21:8: warning: variable 'b' set but not used [-Wunused-but-set-variable]
  int a,b; a = read(); b = read();
        ^
foo.cc:30:3: warning: 'rec' may be used uninitialized in this function [-Wmaybe-uninitialized]
   if(i!=rec){
   ^~

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