What I would do is a quite simple test. Once you have validated you input like you have done, you can just test to see if the inputted number can be divided by 0.5 and the result is a number without fractions. For example, if the user inputted 2.5 - you divide it by 0.5 and you get 5. that is OK. If the user inputted 2.3, and you divide by 0.5 - you get a fraction, which is not OK.
Follow this suggestion of code:
Double input = Double.valueOf(txtLdays.getText.toString());
if (input % 0.5 == 0){ //Check if you number divides by 0.5 with no fraction
//Your number divides by 0.5, input is ok
} else {
//Your number does not divide by 0.5, please re-enter number
}
Hope this helped!