foo.c: In function 'main':
foo.c:17:6: warning: unused variable 'a' [-Wunused-variable]
   17 |  int a[13]={0,31,28,31,30,31,30,31,31,30,31,30,31};
      |      ^
foo.c: In function 'yf':
foo.c:14:1: warning: control reaches end of non-void function [-Wreturn-type]
   14 | }
      | ^
foo.c: In function 'main':
foo.c:18:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   18 |  scanf("%d %d",&year,&mon);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~

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