do{
document.writeln("The number is not " + loop_index + ".")
loop_index++
}while(loop_index != end_number)

document.writeln("However, the number <I>is</I> " + loop_index + ".")

</SCRIPT>

The part to note is the

do{
.......

}while(loop_index != end_number)

the conditional comes at the end of the do while loop