foo.cc: In function 'int main()':
foo.cc:11:7: 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:12:36: 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", a + i);
                               ~~~~~^~~~~~~~~~~~~

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