#include <bits/stdc++.h>
double f[120][120000];
int g[120];
double s;
int i, j, m, n, t;
int main() {
  scanf("%d%d", &n, &m);
  if (m == 1) {
    printf("1");
    return 0;
  }
  for (i = 1; i <= n; i++) scanf("%d", &g[i]);
  f[0][0] = 1, f[0][1] = -1;
  for (i = 0; i < n; i++)
    for (j = 0; j <= n * m; j++) {
      f[i][j] = f[i][j - 1] + f[i][j];
      f[i + 1][j + 1] = f[i + 1][j + 1] + f[i][j] / (m - 1);
      f[i + 1][j + g[i + 1]] = f[i + 1][j + g[i + 1]] - f[i][j] / (m - 1);
      f[i + 1][j + g[i + 1] + 1] =
          f[i + 1][j + g[i + 1] + 1] + f[i][j] / (m - 1);
      f[i + 1][j + m + 1] = f[i + 1][j + m + 1] - f[i][j] / (m - 1);
    }
  for (i = 0; i <= n * m; i++) f[n][i] = f[n][i - 1] + f[n][i];
  for (i = 1; i <= n; i++) t = t + g[i];
  for (i = 0; i < t; i++) s = s + f[n][i];
  printf("%.15lf", s * (m - 1) + 1);
  return 0;
}
