foo.cc: In function 'int main()':
foo.cc:25:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d%d%d%d",&n,&c1,&c2,&c3);
 ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:27:6: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
 scanf("%d%d",a+i,b+i);
 ~~~~~^~~~~~~~~~~~~~~~

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