foo.cc: In function 'int ask(int, int)':
foo.cc:47:9: warning: unused variable 'o' [-Wunused-variable]
     int o,res;
         ^
foo.cc: In function 'int main()':
foo.cc:67:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d%d%d%d",&n,&k,&l,&r);
     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:72:12: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
       scanf("%d",&x);
       ~~~~~^~~~~~~~~

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