import java.io.BufferedInputStream;
import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.util.*;

public class _475 {
	public static void main(String[] args) throws IOException {
		new _475().solve();
	}

	void solve() throws IOException {
		try (final Scanner in = new Scanner(System.in)) {
			int n = in.nextInt();
			long S = in.nextInt();
			int wid = in.nextInt();
			long[] a = new long[n];
			for (int i = 0; i < n; i++) {
				a[i] = in.nextInt();
			}
			a[wid] += 100 * S;
			int[] cnt = new int[n];
			for (int i = 0; i < n; i++) if (i != wid) {
				int low = 0, high = n;
				while (high - low > 1) {
					int mid = (low + high) / 2;
					long x = a[i] + (long)(50 * S + 50 * S / (0.8 + 0.2 * mid));
					if (x > a[wid]) {
						low = mid;
					} else {
						high = mid;
					}
				}
				cnt[low]++;
//				dump(i, low, p);
			}

			for (int i = 1; i < n; i++) {
				logfact[i] = logfact[i-1] + Math.log(i);
			}
			
			double ans = 1;
			int s = 0;
			for (int i = 0; i < n; i++) {
				// sCi/n-1Ci
				double p = s < i ? 0 : prob(n-1, s, i);
				s += cnt[i];
				ans *= p;
//				dump(p, s);
			}
			System.out.printf("%.10f\n", ans);
		}
	}

	double[] logfact = new double[100000];
	double logcomb(int n, int r) {
		return logfact[n] - logfact[r] - logfact[n-r];
	}
	double prob(int n, int s, int i) {
		return Math.exp(logcomb(s, i) - logcomb(n, i));
	}
	
	// for debug
	static void dump(Object... o) {
		System.err.println(Arrays.deepToString(o));
	}
}