#include <bits/stdc++.h>
int n, m, r[105], s;
double dp[105][100005], ans;
int main() {
  scanf("%d %d", &n, &m);
  for (int i = 1; i <= n; i++) {
    scanf("%d", &r[i]);
    s += r[i];
  }
  for (int i = 0; i <= m * n; i++) dp[0][i] = m - 1;
  for (int i = 1; i <= n; i++) {
    for (int j = i; j <= m * i; j++) {
      if (j >= m + 1)
        dp[i][j] = (dp[i - 1][j - 1] - dp[i - 1][j - r[i]] +
                    dp[i - 1][j - r[i] - 1] - dp[i - 1][j - m - 1]) /
                   (m - 1);
      else if (j >= r[i] + 1)
        dp[i][j] =
            (dp[i - 1][j - 1] - dp[i - 1][j - r[i]] + dp[i - 1][j - r[i] - 1]) /
            (m - 1);
      else if (j >= r[i])
        dp[i][j] = (dp[i - 1][j - 1] - dp[i - 1][j - r[i]]) / (m - 1);
      else
        dp[i][j] = dp[i - 1][j - 1] / (m - 1);
    }
    for (int j = 1; j <= m * n; j++) dp[i][j] += dp[i][j - 1];
  }
  printf("%.16lf\n", dp[n][s - 1] + 1);
  scanf("\n");
  return 0;
}
