#include <bits/stdc++.h>
using namespace std;
const int N = 109;
const int M = 1009;
int n, m, a[N], s;
double f[N][M * N], sum[N * M];
int min(int x, int y) { return x < y ? x : y; }
int main() {
  scanf("%d%d", &n, &m);
  s = 0;
  double d = 1.0 / (m - 1);
  for (int i = 1; i <= n; i++) scanf("%d", &a[i]), s += a[i];
  memset(f, 0, sizeof(f));
  memset(sum, 0, sizeof(sum));
  f[0][0] = 1.0;
  sum[0] = 1.0;
  for (int i = 1; i <= n; i++) {
    for (int j = 1; j <= i * m; j++) {
      int l = max(i - 1, j - m), r = min((i - 1) * m, j - 1);
      f[i][j] = sum[r] * d;
      if (l) f[i][j] -= sum[l - 1] * d;
      if (j - a[i] >= l && j - a[i] <= r) f[i][j] -= f[i - 1][j - a[i]] * d;
    }
    sum[0] = 0;
    for (int j = 1; j <= min(i * m, s); j++) sum[j] = sum[j - 1] + f[i][j];
  }
  double p = 0;
  for (int j = n; j <= s - 1; j++) p += f[n][j];
  printf("%.12lf", p * (m - 1) + 1);
  return 0;
}
