foo.cc: In function 'int main()':
foo.cc:10:10: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     scanf("%d%d%d%d",&n,&k,&a,&b);
     ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~
foo.cc:11:31: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
     for(int i=1;i<=n;++i)scanf("%d",&c[i]);
                          ~~~~~^~~~~~~~~~~~

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