#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=a;i<n;i++)
#define per(i,a,n) for (int i=n-1;i>=a;i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef basic_string<int> BI;
typedef long long ll;
typedef pair<int,int> PII;
typedef double db;
mt19937 mrand(random_device{}()); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
// head

const int N=20;
db comb[N][N],fac,way,tot;
int cnt[N],a[N];
int k,m;
void dfs(int d,int s) {
	if (d==m) {
		if (s<k) return;
		db R=1,L=0;
		rep(i,0,m) R*=comb[i][cnt[i]];
		rep(i,0,m) if (cnt[i]>0) {
			L+=comb[i][cnt[i]-1]/comb[i][cnt[i]];
		}
		L=L*fac*R;
		tot+=R;
		way+=min(L,R);
	} else {
		rep(i,0,a[d]+1) if (s+i<=k) {
			cnt[d]=i;
			dfs(d+1,s+i);
		} else break;
	}
}
int main() {
	scanf("%d%d",&k,&m);
	fac=1;
	rep(i,1,k) fac*=i;
	rep(i,0,m) {
		scanf("%d",&a[i]);
		comb[i][0]=1;
		rep(j,1,k+1) {
			comb[i][j]=comb[i][j-1]*(a[i]-j+1)/j;
		}
	}
	dfs(0,0);
	printf("%.10f\n",way/tot);
}