foo.c: In function 'counter':
foo.c:17:14: error: expected declaration specifiers or '...' before numeric constant
   17 |  int i,j,cnt(0);
      |              ^
foo.c:23:5: error: 'cnt' undeclared (first use in this function); did you mean 'int'?
   23 |     cnt++;
      |     ^~~
      |     int
foo.c:23:5: note: each undeclared identifier is reported only once for each function it appears in
foo.c: In function 'main':
foo.c:9:2: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
    9 |  scanf("%d%d",&n,&m);
      |  ^~~~~~~~~~~~~~~~~~~
foo.c: In function 'counter':
foo.c:27:1: warning: control reaches end of non-void function [-Wreturn-type]
   27 | }
      | ^
foo.c: In function 'input':
foo.c:33:3: warning: ignoring return value of 'scanf', declared with attribute warn_unused_result [-Wunused-result]
   33 |   scanf("%d",&serial[i]);
      |   ^~~~~~~~~~~~~~~~~~~~~~

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