foo.c: In function 'main':
foo.c:21:6: warning: implicit declaration of function 'abs' [-Wimplicit-function-declaration]
   21 |  sum=abs(sum);
      |      ^~~
foo.c:9:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    9 |  scanf("%d%d%d%d",&month1,&day1,&month2,&day2);
      |  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

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