#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)




int N, S, ID;
int A[101010], MY;
//-----------------------------------------------------------------
typedef long long ll;
ll sc(int r) {
	return 50LL * S + 500LL * S / (8 + 2 * r);
}
//-----------------------------------------------------------------
int main() {
	cin >> N >> S >> ID;
	rep(i, 0, N) {
		if (i < ID) scanf("%d", &A[i]);
		else if (i == ID) scanf("%d", &MY);
		else scanf("%d", &A[i - 1]);
	}
	N--;

	sort(A, A + N, greater<int>());
	MY += 100 * S;
	
	// 1位が最下位で勝てないなら無理
	if (MY < A[0] + sc(N)) {
		printf("0\n");
		return 0;
	}

	double ans = 1;

	rep(i, 0, N) {
		int lo = 0, hi = N;
		while (lo + 1 != hi) {
			int mi = (lo + hi) / 2;
			if (A[i] + sc(mi) <= MY) hi = mi;
			else lo = mi;
		}

		// hi位以下であれば勝てる
		int ok = N - hi + 1;
		if (ok < i) {
			printf("0\n");
			return 0;
		}
		ans *= (ok - i) * 1.0 / (N - i);
	}
	printf("%.10f\n", ans);
}