#include <bits/stdc++.h>
using namespace std;
const int N = 512345, lgN = 22, rootN = 5001;
const double eps = 1e-9;
double dp[112][112345];
int x[112];
int main() {
  int n, m;
  scanf("%d %d", &n, &m);
  int s = 0;
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &x[i]);
    s += x[i];
  }
  if (m == 1) {
    printf("%0.15lf\n", 1.0f);
    return 0;
  }
  dp[0][0] = 1.0;
  for (int i = 1; i <= n; ++i) {
    double sum = dp[i - 1][0];
    for (int j = 1; j <= n * m; ++j) {
      if (j > m) sum -= dp[i - 1][j - m - 1];
      dp[i][j] = sum;
      if (j >= x[i]) dp[i][j] -= dp[i - 1][j - x[i]];
      dp[i][j] /= (m - 1);
      sum += dp[i - 1][j];
    }
  }
  double y = 0;
  for (int j = 0; j < s; ++j) y += dp[n][j];
  double ans = 0.0;
  for (int k = 0; k < m; ++k) {
    double cc = 1.0;
    for (int j = 1; j <= k; ++j) {
      cc *= (m - j);
      cc /= j;
    }
    double cur = 1.0;
    for (int i = 0; i < k; ++i) cur *= y;
    for (int i = k + 1; i < m; ++i) cur *= (1.0 - y);
    ans += cc * cur * (k + 1);
  }
  printf("%0.15lf\n", ans);
  return 0;
}
