foo.c: In function 'F':
foo.c:9:6: warning: unused variable 'i' [-Wunused-variable]
  int i, j;
      ^
foo.c: In function 'mul':
foo.c:27:6: warning: unused variable 't' [-Wunused-variable]
  int t;
      ^
foo.c:26:6: warning: unused variable 'i' [-Wunused-variable]
  int i;
      ^
foo.c: In function 'main':
foo.c:51:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d %d", &x1, &n1);
  ^~~~~~~~~~~~~~~~~~~~~~~~
foo.c:52:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
  scanf("%d %d", &x2, &n2);
  ^~~~~~~~~~~~~~~~~~~~~~~~

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