#include <bits/stdc++.h>
using namespace std;
const int N = 55;
int n, m, a[N];
double C[N][N], F[N][N][N];
inline int read() {
  int x = 0, f = 1;
  char ch;
  for (ch = getchar(); !isdigit(ch); ch = getchar())
    if (ch == '-') f = -1;
  for (; isdigit(ch); ch = getchar()) x = x * 10 + ch - '0';
  return x * f;
}
int main() {
  n = read(), m = read();
  C[0][0] = 1;
  for (int i = 1; i <= n; ++i) {
    for (int j = 0; j <= i; ++j) C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
  }
  for (int i = 1; i <= m; ++i) a[i] = read();
  F[0][n][0] = 1;
  for (int i = 1; i <= m; ++i) {
    for (int j = 0; j <= n; ++j) {
      for (int k = 0; k <= j; ++k) {
        int len = k / a[i] + (k % a[i] > 0);
        for (int t = 0; t <= n; ++t) {
          F[i][j - k][max(len, t)] +=
              F[i - 1][j][t] * C[j][k] * pow(1.0 / m, k);
        }
      }
    }
  }
  double ans = 0.0;
  for (int i = 1; i <= n; ++i) ans += F[m][0][i] * i;
  printf("%.18lf\n", ans);
  return ~~(0 ^ 0 ^ 0);
}
