#include <bits/stdc++.h>
using namespace std;
inline int read() {
  int ret = 0, p = 1;
  char ch = getchar();
  for (; !isdigit(ch); ch = getchar())
    if (ch == '-') p = -1;
  for (; isdigit(ch); ch = getchar()) ret = ret * 10 + ch - '0';
  return ret * p;
}
int n, m;
int a[100 + 3];
double f[100000 + 3][2];
double sum[100000 + 3];
int main() {
  n = read();
  m = read();
  for (int i = 1; i <= n; i++) a[i] = read();
  if (m == 1) {
    puts("1");
    return 0;
  }
  for (int i = 1; i <= m; i++)
    if (i != a[1]) f[i][1] = (double)1 / (m - 1);
  int tmp, temp = 1;
  for (int i = 2; i <= n; i++) {
    tmp = temp, temp = !temp;
    for (int j = 1; j <= n * m; j++) sum[j] = sum[j - 1] + f[j][tmp];
    for (int j = 1; j <= i * m; j++) {
      if (j > a[i] && j > m)
        f[j][temp] =
            (double)(sum[j - 1] - sum[j - m - 1] - f[j - a[i]][tmp]) / (m - 1);
      else if (j > a[i])
        f[j][temp] = (double)(sum[j - 1] - f[j - a[i]][tmp]) / (m - 1);
      else
        f[j][temp] = (double)sum[j - 1] / (m - 1);
    }
  }
  double ans = 0;
  int t = 0;
  for (int i = 1; i <= n; i++) t += a[i];
  for (int i = 1; i < t; i++) ans += f[i][temp];
  ans = ans * (m - 1) + 1;
  printf("%.10lf", ans);
  return 0;
}
