#include <bits/stdc++.h>
using namespace std;
typedef long long LL;
typedef pair<int, int> P;
template<typename T> inline void chkmin(T &a, const T &b) { a = a < b ? a : b; }
template<typename T> inline void chkmax(T &a, const T &b) { a = a > b ? a : b; }

int n, C, len[10], sum[10];
LL f[32][305];

int main() {
	scanf("%d%d", &n, &C);
	for (int i = 0; i < n; i++) scanf("%d", len + i);
	sort(len, len + n, greater<int>());
	long double ans = 0;
	for (int i = 1, m = 0; i <= n; i++) {
		for (int j = 0; j < n; j++) sum[j] += len[j];
		m += C;
		memset(f, 0, sizeof(f));
		f[0][sum[0]] = 1;
		for (int o = 0; o < m; o++)
		for (int l = 1; l < n; l++)
		for (int j = 0; j < 1 << (n - 1); j++) if (~j >> (l - 1) & 1)
		for (int k = o; k <= m; k++) if (f[j][k])
			f[j | 1 << (l - 1)][max(min(m, o + sum[l]), k)] += f[j][k];
		long double y = f[(1 << (n - 1)) - 1][m];
		for (int j = 1; j <= n; j++) if (j != i) y /= j - i;
		ans += y;
	}
	if (~n & 1) ans = -ans;
	for (int i = 1; i < n; i++) ans /= C;
	printf("%.18Lf\n", ans);
	return 0;
}