#include <bits/stdc++.h>
using namespace std;
double dp[105][105 * 1005];
int a[105];
int main(void) {
  int n, m;
  scanf("%d%d", &n, &m);
  int sum = 0;
  for (int i = 0; i < n; ++i) {
    scanf("%d", &a[i]);
    sum += a[i];
  }
  if (m == 1) {
    printf("%.10f\n", 1.0);
    return 0;
  }
  dp[0][0] = m - 1;
  for (int i = 1; i <= n; ++i) {
    double sum = 0;
    for (int j = i; j <= i * m; ++j) {
      sum += dp[i - 1][j - 1] / (m - 1);
      if (j - 1 - m >= 0) sum -= dp[i - 1][j - m - 1] / (m - 1);
      dp[i][j] = sum;
      if (j - a[i - 1] >= 0) dp[i][j] -= dp[i - 1][j - a[i - 1]] / (m - 1);
    }
  }
  double ans = 1;
  for (int i = 0; i < sum; i++) ans += dp[n][i];
  printf("%.10f\n", ans);
  return 0;
}
