#include <bits/stdc++.h>
using std::max;
int n, m, a[55];
double dp[55][55][55], C[55][55];
void init() {
  memset(C, 0, sizeof(C));
  C[0][0] = 1;
  for (int i = 1; i <= 50; i++) {
    C[i][0] = 1;
    for (int j = 1; j <= i; j++) {
      C[i][j] = C[i - 1][j - 1] + C[i - 1][j];
    }
  }
}
int main() {
  init();
  scanf("%d%d", &n, &m);
  for (int i = 1; i <= m; i++) {
    scanf("%d", &a[i]);
  }
  memset(dp, 0, sizeof(dp));
  for (int i = 0; i <= n; i++) dp[0][0][i] = i;
  for (int i = 1; i <= m; i++) {
    for (int j = 0; j <= n; j++) {
      for (int k = 0; k <= n; k++) {
        for (int c = 0; c <= j; c++) {
          int p = max(k, (c - 1) / a[i] + 1);
          dp[i][j][k] += 1.0 * dp[i - 1][j - c][p] * pow(i - 1, j - c) *
                         C[j][c] / pow(i, j);
        }
      }
    }
  }
  printf("%.12lf\n", dp[m][n][0]);
  return 0;
}
