#include <bits/stdc++.h>
using namespace std;
const int N = 105;
const int M = 1005;
int n, m, x[N], s;
double p[N * M], tmp[N * M];
int main() {
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= n; ++i) scanf("%d", &x[i]);
  for (int i = 1; i <= n; ++i) s += x[i];
  if (m == 1) {
    puts("1.0000000000");
    return 0;
  }
  p[0] = 1;
  for (int i = 1; i <= n; ++i) {
    double now = 0;
    for (int j = 1; j < s; ++j) {
      now += p[j - 1];
      if (x[i] <= j) now -= p[j - x[i]];
      if (x[i] < j) now += p[j - x[i] - 1];
      if (m < j) now -= p[j - m - 1];
      tmp[j] = now / (m - 1);
    }
    for (int j = 0; j <= s; ++j) p[j] = tmp[j];
    copy(tmp, tmp + s, p);
  }
  double ans = 0;
  for (int i = 1; i <= s; ++i) ans += p[i];
  ans = ans * (m - 1) + 1;
  printf("%.10f\n", ans);
  return 0;
}
