/in/foo.c: In function 'main':
/in/foo.c:19:2: warning: implicit declaration of function 'gets' [-Wimplicit-function-declaration]
  gets(s1);
  ^~~~
/in/foo.c:23:2: warning: implicit declaration of function 'printf' [-Wimplicit-function-declaration]
  printf("%d %d",cha1,cha2);
  ^~~~~~
/in/foo.c:23:2: warning: incompatible implicit declaration of built-in function 'printf'
/in/foo.c:23:2: note: include '<stdio.h>' or provide a declaration of 'printf'
/tmp/ccaFwjZ9.o: In function `main':
foo.c:(.text.startup+0x17): warning: the `gets' function is dangerous and should not be used.