/in/foo.c: In function 'Diff':
/in/foo.c:10:17: error: 's1' undeclared (first use in this function)
   if(s[i]<='z'&&s1[i]>='a')
                 ^~
/in/foo.c:10:17: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:6: warning: unused variable 'n' [-Wunused-variable]
  int n=strlen(s);
      ^
/in/foo.c: In function 'main':
/in/foo.c:29:2: 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:30:2: 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:24:11: warning: variable 'n2' set but not used [-Wunused-but-set-variable]
  int i,n1,n2;
           ^~
/in/foo.c:24:8: warning: variable 'n1' set but not used [-Wunused-but-set-variable]
  int i,n1,n2;
        ^~
/in/foo.c:24:6: warning: unused variable 'i' [-Wunused-variable]
  int i,n1,n2;
      ^