/in/foo.c: In function 'main':
/in/foo.c:6:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(a);
  ^~~~
In file included from /in/foo.c:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.c:7:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(b);
  ^~~~
In file included from /in/foo.c:1:0:
/usr/include/stdio.h:640:14: note: declared here
 extern char *gets (char *__s) __wur __attribute_deprecated__;
              ^~~~
/in/foo.c:23:4: error: 'i' undeclared (first use in this function)
  b[i]=0;
    ^
/in/foo.c:23:4: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:5:6: warning: unused variable 'n' [-Wunused-variable]
  int n=26,k;
      ^