foo.c: In function 'Merge':
foo.c:16:6: warning: unused variable 'N' [-Wunused-variable]
  int N;
      ^
foo.c:15:6: warning: unused variable 'j' [-Wunused-variable]
  int j;
      ^
foo.c: In function 'main':
foo.c:32:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d %d %d", &n1, &n2, &n3);
  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
foo.c:38:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &a1[i]);
   ^~~~~~~~~~~~~~~~~~~
foo.c:44:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &a2[i]);
   ^~~~~~~~~~~~~~~~~~~
foo.c:47:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   scanf("%d", &a3[i]);
   ^~~~~~~~~~~~~~~~~~~

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