import java.util.Scanner;

public class FinalExamMark {
	public static void main(String[] args) {
		Scanner input = new Scanner(System.in);

		while (true)	{
			//get average the user wants
			System.out.print("Enter the desired average: ");
			int d = input.nextInt();

			//if input average is negative, exit the program
			if (d < 0)	{
				System.out.println("Goodbye.");
				System.exit(1);
			}

			//get number of tests including the final
			System.out.print("Enter the number of tests (including the final): ");
			int k = input.nextInt();
			//use nextLine to avoid skipping weights entry
			input.nextLine();

			//get the test weights
			System.out.print("Enter the test weights: ");
			String weightsString = input.nextLine();

			int[] weights = stringToArray(weightsString, k);

			//get the test grades before the exam (k - 1)
			System.out.print("Enter the previous test grades (out of 100): ");
			String gradesString = input.nextLine();

			int[] gradesInt = stringToArray(gradesString, (k - 1));
			double[] grades = new double[gradesInt.length];

			//convert grades to a double array
			for (int i = 0; i < (k - 1); i++)	{
				grades[i] = gradesInt[i];
			}

			//debug lines
			//System.out.println(Arrays.toString(weights));
			//System.out.println(Arrays.toString(grades));

			//divide the marks for each test by 100
			for (int i = 0; i < grades.length; i++)	{
				grades[i] = grades[i] / 100;
			}

			//add the marks for each test up
			double totalMarks = 0;
			for (int i = 0; i < grades.length; i++)	{
				totalMarks += grades[i];
			}

			//divide totalMarks by the number of tests
			totalMarks = totalMarks / (k - 1);
			//multiply that value by 100 to get the mark thus far
			totalMarks = totalMarks * 100;

			double finalWeight = weights[weights.length - 1] / 100;

			double finalMark = (d - ((1 - finalWeight) * totalMarks)) / finalWeight;

			System.out.println(finalMark);
		}
	}

	public static int[] stringToArray(String string, int arrayLength)	{
		//split all the numbers up into an array
		String[] stringArray = string.split("\\s+");
		int[] array = new int[arrayLength];

		//convert the string array to an int array
		for (int i = 0; i < arrayLength; i++)    {
            array[i] = Integer.parseInt(stringArray[i]);
        }

		return array;
	}
}