#include <bits/stdc++.h>
using namespace std;
int n, m, a[60];
double c[60][60], dp[60][60][60];
int main() {
  cin >> n >> m;
  for (int i = 1; i <= m; i++) cin >> a[i];
  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];
  }
  dp[0][0][0] = 1;
  for (int i = 1; i <= m; i++) {
    for (int j = 0; j <= n; j++) {
      for (int l = 0; l <= j; l++) {
        for (int p = (l - 1) * a[i] + 1; p <= l * a[i] && p <= j; p++)
          for (int k = 0; k <= l; k++)
            dp[i][j][l] += dp[i - 1][j - p][k] * c[n - j + p][p];
        for (int k = 0; k <= (l - 1) * a[i] && k <= j; k++)
          dp[i][j][l] += dp[i - 1][j - k][l] * c[n - j + k][k];
      }
    }
  }
  double a = 0, b = 0;
  for (int l = 0; l <= n; l++) a += l * dp[m][n][l], b += dp[m][n][l];
  printf("%.20lf", a / b);
  return 0;
}
