#include <bits/stdc++.h>
using namespace std;
int n, m, a[200], sum;
double dp[110][100010], s[110][100010];
int main() {
  scanf("%d%d", &n, &m);
  int nm = n * m;
  for (int i = (1); i <= (n); i++) scanf("%d", &a[i]);
  if (m == 1) {
    puts("1.0");
    return 0;
  }
  for (int i = (1); i <= (n); i++) sum += a[i];
  dp[0][0] = 1;
  for (int j = (0); j <= (nm); j++) s[0][j] = 1;
  for (int i = (1); i <= (n); i++) {
    for (int j = (1); j <= (m); j++) {
      dp[i][j] = s[i - 1][j - 1];
      if (j >= a[i]) dp[i][j] -= dp[i - 1][j - a[i]];
      dp[i][j] /= m - 1;
      s[i][j] = s[i][j - 1] + dp[i][j];
    }
    for (int j = (m + 1); j <= (nm); j++) {
      dp[i][j] = (s[i - 1][j - 1] - s[i - 1][j - m - 1] - dp[i - 1][j - a[i]]) /
                 (m - 1);
      s[i][j] = s[i][j - 1] + dp[i][j];
    }
  }
  printf("%.9lf\n", 1 + s[n][sum - 1] * (m - 1));
  return 0;
}
