foo.c: In function 'Concat1':
foo.c:11:8: error: 'i' undeclared (first use in this function)
   11 |  s1[n1+i]=0;
      |        ^
foo.c:11:8: note: each undeclared identifier is reported only once for each function it appears in
foo.c: In function 'Concat':
foo.c:18:8: error: 'i' undeclared (first use in this function)
   18 |  s1[n1+i-2]=0;
      |        ^
foo.c: In function 'main':
foo.c:23:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   23 |  char s1[9999]; gets(s1);
      |  ^~~~
In file included from /usr/include/stdio.h:867,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.c:24:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
   24 |  char s2[9999];  gets(s2);
      |  ^~~~
In file included from /usr/include/stdio.h:867,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:235:1: note: declared here
  235 | gets (char *__str)
      | ^~~~
foo.c:23:17: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
   23 |  char s1[9999]; gets(s1);
      |                 ^~~~~~~~
foo.c:24:18: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
   24 |  char s2[9999];  gets(s2);
      |                  ^~~~~~~~

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