package yukicoder;

import java.util.*;

public class Q475 {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int N = sc.nextInt();
		long S = sc.nextLong();
		int writerId = sc.nextInt();
		long[] a = new long[N];
		for (int i = 0; i < N; ++i) {
			a[i] = sc.nextLong();
		}
		long writerPoint = a[writerId] + 100 * S;

		long[] p = new long[N + 1];
		for (int i = 1; i < N; ++i) {
			p[i] = (long) (50 * S + 50 * S / (0.8 + 0.2 * i));
		}

		int[] R = new int[N + 1];
		for (int i = 0; i < N; ++i) {
			if (i == writerId)
				continue;
			if (p[N] + a[i] > writerPoint) {
				System.out.println(0);
				return;
			}
			int l = 0;
			int r = N;
			while (r - l > 1) {
				int mid = (l + r) / 2;
				if (p[mid] + a[i] <= writerPoint) {
					r = mid;
				} else {
					l = mid;
				}
			}
			++R[r];
		}
		double pb = 1;
		double s = 0;
		for (int i = 1; i <= N - 1; ++i) {
			s += R[i];
			pb *= s;
			--s;
		}
		for (int i = N - 1; i >= 1; --i) {
			pb /= i;
		}
		System.out.println(pb);

	}

	static void tr(Object... objects) {
		System.out.println(Arrays.deepToString(objects));
	}
}