#include <bits/stdc++.h>
using namespace ::std;
const long double PI = acos(-1);
const long long MOD = 1000000000 + 7;
long long gcd(long long a, long long b) { return b == 0 ? a : gcd(b, a % b); }
long long add(long long a, long long b, long long m = MOD) {
  return (a % m + b % m + 2 * m) % m;
}
long long mul(long long a, long long b, long long m = MOD) {
  return ((a % m + m) * (b % m + m)) % m;
}
long long pow_mod(long long a, long long b, long long m = MOD) {
  long long res = 1LL;
  a = a % m;
  while (b) {
    if (b & 1) res = mul(res, a, m);
    b >>= 1;
    a = mul(a, a, m);
  }
  return res;
}
long long fastexp(long long a, long long b) {
  long long res = 1LL;
  while (b) {
    if (b & 1) res = res * a;
    b >>= 1;
    a *= a;
  }
  return res;
}
int gcdExtendido(int a, int b, int *x, int *y) {
  if (a == 0) {
    *x = 0;
    *y = 1;
    return b;
  }
  int x1, y1;
  int gcd = gcdExtendido(b % a, a, &x1, &y1);
  *x = y1 - (b / a) * x1;
  *y = x1;
  return gcd;
}
int modInverso(int a, int m) {
  int x, y;
  int g = gcdExtendido(a, m, &x, &y);
  if (g != 1)
    return -1;
  else
    return (x % m + m) % m;
}
const int MAX = 1000000 + 5;
const int N = 100 + 5;
int n, m;
int v[N];
long double memo[2][MAX];
int main() {
  scanf("%d %d", &(n), &(m));
  int S = 0;
  for (int i = 1; i <= n; i++) {
    scanf("%d", &(v[i]));
    S += v[i];
  }
  if (m == 1) {
    puts("1.0000000000000000");
    return 0;
  }
  memo[0][0] = m - 1;
  for (int i = 0; i < n; i++) {
    int next = (i + 1) & 1;
    int act = i & 1;
    for (int j = 1; j <= n * m; j++) {
      memo[act][j] += memo[act][j - 1];
    }
    for (int j = 0; j <= n * m; j++) {
      memo[next][j] = 0.0;
    }
    int next_chosen = v[i + 1];
    for (int j = 0; j <= i * m; j++) {
      memo[next][j + 1] += memo[act][j] / (m - 1);
      memo[next][j + m + 1] -= memo[act][j] / (m - 1);
      memo[next][j + next_chosen] -= memo[act][j] / (m - 1);
      memo[next][j + next_chosen + 1] += memo[act][j] / (m - 1);
    }
  }
  for (int j = 1; j <= n * m; j++) {
    memo[n & 1][j] += memo[n & 1][j - 1];
  }
  long double ans = 0.0;
  for (int j = 0; j < S; j++) {
    ans += memo[n & 1][j];
  }
  ans += 1.0;
  printf("%.16Lf\n", ans);
  return 0;
}
