foo.c: In function 'main':
foo.c:7:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d%d", &na, &nb);
  ^~~~~~~~~~~~~~~~~~~~~~~
foo.c:9:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &a[i]);
   ^~~~~~~~~~~~~~~~~~
foo.c:11:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &b[j]);
   ^~~~~~~~~~~~~~~~~~

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