import std.conv, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.container, std.math, std.numeric, std.range, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken().to!int; }
long readLong() { return readToken().to!long; }
real readReal() { return readToken().to!real; }

void chmin(T)(ref T t, in T f) { if (t > f) t = f; }
void chmax(T)(ref T t, in T f) { if (t < f) t = f; }

int binarySearch(T)(in T[] as, in bool delegate(T) test) { int low = -1, upp = cast(int)(as.length); for (; low + 1 < upp; ) { int mid = (low + upp) >> 1; (test(as[mid]) ? low : upp) = mid; } return upp; }
int lowerBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a < val)); }
int upperBound(T)(in T[] as, in T val) { return as.binarySearch((T a) => (a <= val)); }


int N;
long S;
int W;
long[] A;

void main() {
  try {
    for (; ; ) {
      N = readInt();
      S = readLong();
      W = readInt();
      A = new long[N];
      foreach (i; 0 .. N) {
        A[i] = readLong();
      }
      
      auto pts = new long[N - 1];
      foreach (i; 0 .. N - 1) {
        // floor(50 S + 50 S / (0.8 + 0.2 (1 + i)))
        pts[i] = 50 * S + 10 * 50 * S / (8 + 2 * (1 + i));
      }
      pts.sort;
      debug {
        writeln("pts = ", pts);
      }
      
      int[] xs;
      foreach (i; 0 .. N) {
        if (i != W) {
          xs ~= pts.binarySearch((long p) => (A[i] + p <= A[W] + 100 * S));
        }
      }
      debug {
        writeln("xs = ", xs);
      }
      xs.sort;
      
      real ans = 1.0;
      foreach (i; 0 .. N - 1) {
        ans *= max(xs[i] - i, 0);
        ans /= (N - 1 - i);
      }
      writefln("%.20f", ans);
    }
  } catch (EOFException e) {
  }
}