/in/foo.c: In function 'main':
/in/foo.c:6:10: warning: implicit declaration of function 'jud' [-Wimplicit-function-declaration]
  while (!jud(n))
          ^~~
/in/foo.c:8:7: warning: implicit declaration of function 'change' [-Wimplicit-function-declaration]
   n = change(n);
       ^~~~~~