/in/foo.c: In function 'main':
/in/foo.c:6:2: warning: 'gets' is deprecated [-Wdeprecated-declarations]
  gets(s);
  ^~~~
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:7:2: error: unknown type name 'bool'
  bool flag=false; 
  ^~~~
/in/foo.c:7:12: error: 'false' undeclared (first use in this function)
  bool flag=false; 
            ^~~~~
/in/foo.c:7:12: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:13:13: error: 'true' undeclared (first use in this function)
    if(flag==true) ei=i-1;
             ^~~~