foo.c: In function 'main':
foo.c:19:2: warning: implicit declaration of function 'gets' [-Wimplicit-function-declaration]
   19 |  gets(s1);
      |  ^~~~
foo.c:23:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
   23 |  printf("%d %d",cha1,cha2);
      |  ^~~~~~
foo.c:23:2: warning: incompatible implicit declaration of built-in function 'printf'
foo.c:2:1: note: include '<stdio.h>' or provide a declaration of 'printf'
    1 | #include<string.h>
  +++ |+#include <stdio.h>
    2 | int Diff(char s[])
/usr/bin/ld: /tmp/ccxCBbci.o: in function `main':
foo.c:(.text.startup+0x49): warning: the `gets' function is dangerous and should not be used.

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