#include <bits/stdc++.h>
using namespace std;
const int N = 110, M = 1010;
int n, m;
int x[N];
double f[N][N * M], sum[N * M];
int main() {
  scanf("%d%d", &n, &m);
  int cnt = 0;
  for (int i = 1; i <= n; ++i) {
    scanf("%d", &x[i]);
    cnt += x[i];
  }
  f[0][0] = 1;
  for (int i = 1; i <= n; ++i) {
    memset(sum, 0, sizeof(sum));
    sum[0] = f[i - 1][0];
    for (int j = 1; j <= m * i && j < cnt; ++j)
      sum[j] = sum[j - 1] + f[i - 1][j];
    for (int j = i; j <= m * i && j < cnt; ++j) {
      f[i][j] = sum[j - 1];
      if (j - m - 1 > 0) f[i][j] -= sum[j - m - 1];
      if (j - x[i] >= 0) f[i][j] -= f[i - 1][j - x[i]];
      f[i][j] /= m - 1;
    }
  }
  double res = 0;
  for (int i = n; i < cnt; ++i) res += f[n][i];
  res = res * (m - 1) + 1;
  printf("%.16f\n", res);
  return 0;
}
