#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e5 + 10;
const int maxint = 0x3f3f3f3f;
const int mod = 1e9 + 7;
double a[110][maxn], b[110][maxn], ans;
int n, m, f[110], sum;
int main() {
  int i, j;
  cin >> n >> m;
  for (i = 1; i <= n; i++) {
    cin >> f[i];
    sum += f[i];
  }
  if (m == 1) {
    cout << "1\n";
    return 0;
  }
  a[0][0] = 1.0;
  b[0][0] = 1.0;
  for (i = 1; i <= n * m; i++) {
    a[0][i] = 0.0;
    b[0][i] = 1.0;
  }
  for (i = 1; i <= n; i++) {
    for (j = 1; j <= n * m; j++) {
      a[i][j] = (b[i - 1][j - 1] - (j - m - 1 < 0 ? 0 : b[i - 1][j - m - 1]) -
                 (j - f[i] < 0 ? 0 : a[i - 1][j - f[i]])) /
                (double)(m - 1);
      b[i][j] = b[i][j - 1] + a[i][j];
    }
  }
  ans = b[n][sum - 1] * (double)(m - 1) + 1.0;
  printf("%.10lf\n", ans);
  return 0;
}
