foo.cc: In function ‘int month(int)’:
foo.cc:11:1: warning: control reaches end of non-void function [-Wreturn-type]
   11 | }
      | ^
foo.cc: In function ‘int tianshu(int, int, int, int)’:
foo.cc:23:1: warning: control reaches end of non-void function [-Wreturn-type]
   23 | }
      | ^