/in/foo.c: In function 'main':
/in/foo.c:8:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s1);
  ^~~~
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:9:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s2);
  ^~~~
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:7:6: warning: unused variable 'i' [-Wunused-variable]
  int i;
      ^
/in/foo.c: In function 'change':
/in/foo.c:37:15: error: 'k' undeclared (first use in this function)
   if(n1<=n2) {k=(i-1)/2;c[i]=a[n];}
               ^
/in/foo.c:37:15: note: each undeclared identifier is reported only once for each function it appears in