#include <bits/stdc++.h>
using namespace std;
int n, m;
int R[101];
double dp[100100], sum[100100];
int main() {
  scanf("%d%d", &n, &m);
  if (m == 1) {
    puts("1");
    return 0;
  }
  int score = 0;
  dp[0] = 1;
  double f = 1. / (m - 1);
  for (int i = 0; i < n; ++i) {
    scanf("%d", R + i);
    score += R[i];
  }
  for (int i = 0; i < n; ++i) {
    int r = R[i];
    sum[0] = dp[0];
    for (int j = 1; j <= min(score - 1, (i + 1) * m); ++j) {
      sum[j] = sum[j - 1] + dp[j];
    }
    for (int j = min(score - 1, (i + 1) * m); j > 0; --j) {
      dp[j] = sum[j - 1];
      if (j - m - 1 >= 0) dp[j] -= sum[j - m - 1];
      if (j - r >= 0) dp[j] -= dp[j - r];
      dp[j] *= f;
    }
    dp[0] = 0;
  }
  double res = 1;
  for (int i = 0; i < score; ++i) {
    res += dp[i] * (m - 1);
  }
  printf("%.15lf\n", res);
  return 0;
}
