/in/foo.c: In function 'main':
/in/foo.c:31:5: 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:32:5: 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:33:21: warning: implicit declaration of function 'Diff' [-Wimplicit-function-declaration]
     printf("%d %d", Diff(s1), Diff(s2));
                     ^~~~
/tmp/ccte3VXt.o: In function `main':
foo.c:(.text.startup+0x15): warning: the `gets' function is dangerous and should not be used.
foo.c:(.text.startup+0x27): undefined reference to `Diff'
foo.c:(.text.startup+0x33): undefined reference to `Diff'
collect2: error: ld returned 1 exit status