/in/foo.c: In function 'main':
/in/foo.c:5:2: warning: implicit declaration of function 'gets' [-Wimplicit-function-declaration]
  gets(s1);
  ^~~~
/in/foo.c:21:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("%d %d",y1-x1,y2-x2);
  ^~~~~~
/in/foo.c:21:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:21:2: note: include '<stdio.h>' or provide a declaration of 'printf'
/tmp/cc3dKK8p.o: In function `main':
foo.c:(.text.startup+0x17): warning: the `gets' function is dangerous and should not be used.