#include <bits/stdc++.h>
const int N = 100, M = 1000;
int n, m;
double dp[N + 2][N * M + 2];
int main() {
  scanf("%d %d", &n, &m);
  if (m == 1) {
    printf("1.0");
    return 0;
  }
  dp[0][0] = m - 1;
  int sumr = 0;
  for (int i = 1; i <= n; i++) {
    int rank;
    scanf("%d", &rank);
    sumr += rank;
    double sum = 0.0;
    for (int j = 1; j <= n * m; j++) {
      sum += dp[i - 1][j - 1];
      if (j > m) sum -= dp[i - 1][j - m - 1];
      if (j >= rank)
        dp[i][j] = (sum - dp[i - 1][j - rank]) / (m - 1);
      else
        dp[i][j] = sum / (m - 1);
    }
  }
  double ans = 1.0;
  for (int i = 1; i < sumr; i++) ans += dp[n][i];
  printf("%.10lf", ans);
  return 0;
}
