foo.c: In function 'main':
foo.c:7:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    7 |  scanf("%d%d", &n1, &n2);
      |  ^~~~~~~~~~~~~~~~~~~~~~~
foo.c:9:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    9 |   scanf("%d", &a[i]);
      |   ^~~~~~~~~~~~~~~~~~
foo.c:11:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   11 |   scanf("%d", &b[i]);
      |   ^~~~~~~~~~~~~~~~~~
foo.c: In function 'isprime':
foo.c:28:1: warning: control reaches end of non-void function [-Wreturn-type]
   28 | }
      | ^

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