#include <bits/stdc++.h>
using namespace std;
template <class TAT>
void read(TAT &a) {
  static char cc;
  static bool fff;
  while (((cc = getchar()) < '0' || cc > '9') && cc != '-')
    ;
  if (cc == '-')
    fff = 1, a = 0;
  else
    fff = 0, a = cc - 48;
  while ((cc = getchar()) >= '0' && cc <= '9') a = a * 10 + cc - 48;
  if (fff) a = -a;
}
template <class TAT>
void write(TAT a) {
  static char cc[25];
  static int ct;
  if (a == 0) {
    putchar('0');
    return;
  }
  if (a < 0) {
    putchar('-');
    a = -a;
  }
  ct = 0;
  while (a) {
    cc[++ct] = a % 10 + 48;
    a /= 10;
  }
  while (ct) {
    putchar(cc[ct--]);
  }
}
void begin() {
  freopen("input.txt", "r", stdin);
  freopen("output.txt", "w", stdout);
}
void end() {
  fclose(stdin);
  fclose(stdout);
}
template <class TAT>
void Ckmin(TAT &a, const TAT &b) {
  if (a > b) a = b;
}
template <class TAT>
void Ckmax(TAT &a, const TAT &b) {
  if (a < b) a = b;
}
const int maxn = 105;
const int maxm = 1005;
const int maxs = 100005;
int n, m, Ts = 0;
int A[maxn] = {0};
long double f[2][maxs] = {0};
long double Sum[maxs] = {0};
void Init() {
  register int i, j, k;
  read(n), read(m);
  for (i = 1; i <= n; ++i) {
    read(A[i]);
    Ts += A[i];
  }
}
void Solve() {
  register int i, j, k;
  register int nt = 0;
  register long double S = 0;
  if (m == 1) {
    puts("1.00000000000000");
    return;
  }
  f[nt][0] = 1;
  for (i = 1; i <= n; ++i) {
    Sum[1] = f[nt][0];
    for (j = 1; j < Ts; ++j) Sum[j + 1] = Sum[j] + f[nt][j];
    f[nt ^ 1][0] = 0;
    for (j = 1; j < Ts; ++j) {
      f[nt ^ 1][j] = Sum[j] - Sum[j - min(j, m)];
      if (A[i] <= min(j, m)) f[nt ^ 1][j] -= f[nt][j - A[i]];
      f[nt ^ 1][j] /= (long double)(m - 1);
    }
    nt ^= 1;
  }
  for (i = 0; i < Ts; ++i) S += f[nt][i];
  S *= (m - 1);
  S += 1;
  printf("%.18f\n", (double)(S + 1e-18));
}
int main() {
  Init();
  Solve();
  return 0;
}
