(HELP) How to reference a variable outside of scope in C#

I'm learning C#, and my professor asked us to create a program that takes in a user's height and weight, and then calculate bmi. I decided to take it a little further and added in some "input validation" logic. This means that if someone inputs "cat" for their weight, it let the user know that "cat" is not a valid weight.

class MainClass
{
    public static void Main ()
    {
        float userWeight;
        float userHeight;           
        bool weight = true;
        Console.Write ("Weight:  ");
        while (weight) 
        {               
            var inputWeight = (Console.ReadLine ());
            if (!float.TryParse (inputWeight, out userWeight)) {
                Console.WriteLine ("Invalid input");
                Console.Write ("Please try again: ");
            } 
            else 
            {
                weight = false;
            }
        }
        bool height = true;
        Console.Write ("Height:  ");
        while (height) 
        {
            var inputHeight = (Console.ReadLine ());
            if (!float.TryParse (inputHeight, out userHeight)) {
                Console.WriteLine ("Invalid input");
                Console.Write ("Please try again: ");
            } 
            else 
            {
                height = false;
            }
        }
        float bmiHeight = userHeight * userHeight; // error for userHeight
        float bmi = userWeight / bmiHeight * 703;  // error for userWeight
        Console.WriteLine ("You BMI is " + bmi);
    }           
}

The error I get is "use of unassigned local variable..". I know that I am assigning the user variables within IF statements, and that they only persist until the end of that IF statement. My question is, how do I generate a variable in an if statement, and then reference that variable outside of that statement? Certainly I don't have to nest them all, because that seems tedious....