foo.cc: In function 'int main()':
foo.cc:71:7: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d", &N, &M);
  ~~~~~^~~~~~~~~~~~~~~~
foo.cc:74:8: warning: ignoring return value of 'int scanf(const char*, ...)', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d%d%d", &opt, &A, &B);
   ~~~~~^~~~~~~~~~~~~~~~~~~~~~~~