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)); } }