Write a program that asks a user for a weight in pounds (possibly non-integral), and outputs the weight in kilograms. (For the record, 1 pound equals approximately 0.45359 kilograms.)

Your program should repeatedly ask until ensuring that the user correctly entered a nonnegative numeric value as input. As sample session might appear as follows

Enter a weight in pounds: no
That is not a valid number.
Enter a weight in pounds: -5.3
Weights must be nonnegative.
Enter a weight in pounds: 2.5
That is equal to 1.13398 kilograms.