foo.c: In function 'Opp':
foo.c:3:5: warning: type of 'x' defaults to 'int' [-Wimplicit-int]
 int Opp(x)
     ^~~
foo.c: In function 'main':
foo.c:18:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d",&a);
  ^~~~~~~~~~~~~~

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