import java.util.*;
import java.text.DecimalFormat;

public class dice {
	public static void main(String[] args) {
		Scanner s = new Scanner(System.in);
		DecimalFormat fmt = new DecimalFormat("0.00000");
		while (true) {
			int n = s.nextInt();
			if (n == 0) break;
			double[][] answer = new double[n][1001];
			for (int i = 0; i < n; i++) {
				String temp = s.next();
				int up = Integer.parseInt(temp.substring(1));
				if (i == 0) {
					for (int j = 1; j <= up; j++) {
						answer[0][j] = 1.0 / up;
					}
				} else {
					for (int j = 1; j <= up; j++) {
						for (int k = 1; k <= 1000; k++) {
							if (answer[i - 1][k] == 0) continue;
							answer[i][j + k] += answer[i - 1][k] * 1.0 / up;
						}
					}
				}
			}
			int index = s.nextInt();
			System.out.println(fmt.format(answer[n - 1][index]));
		}
	}
}