#include<bits/stdc++.h>
#define Gc() getchar() 
#define Me(x,y) memset(x,y,sizeof(x))
#define Mc(x,y) memcpy(x,y,sizeof(x))
#define d(x,y) ((m)*(x-1)+(y))
#define R(n) (rnd()%(n)+1)
#define Pc(x) putchar(x)
#define LB lower_bound
#define UB upper_bound
#define fi first
#define se second
using namespace std;using ll=long long;using db=double;using lb=long db;using ui=unsigned;using ull=unsigned long long;using pii=pair<int,int>;using LL=__uint128_t;
const int N=100+5,M=N*8+5,K=600+5,mod=998244353,Mod=mod-1;const db eps=1e-6;const int INF=1e9+7;mt19937 rnd(263082);
int k,m,n,A[N],B[N];
db kk,tot,ans,w[N][N];
void calc(){
	db pus=1;
	for(int i=1;i<=m;i++) pus*=w[i][B[i]];
	pus/=tot;
	db aa=0;
	for(int i=1;i<=m;i++) if(B[i]) aa+=kk*B[i]/(A[i]-B[i]+1);
	// cerr<<pus<<' '<<aa<<' '<<B[1]<<' '<<B[2]<<' '<<B[3]<<'\n';
	ans+=pus*min(aa,1.0);
}
void dfs(int x,int k){
	if(x==m) {if(A[x]>=k) B[x]=k,calc();return;}
	for(int i=0;i<=min(k,A[x]);i++) B[x]=i,dfs(x+1,k-i);
}
void Solve(){
	int i,j,h;scanf("%d%d",&k,&m);
	for(i=1;i<=m;i++){
		scanf("%d",&A[i]),n+=A[i];
		w[i][0]=1;
		for(j=1;j<=k;j++){
			w[i][j]=w[i][j-1]*(A[i]-j+1)/j;
		}
	} 
	kk=1;for(i=1;i<k;i++) kk*=i;
	tot=1;for(i=1;i<=k;i++) tot=tot*(n-i+1)/i;
	// cerr<<tot<<'\n';
	dfs(1,k);
	printf("%.9lf\n",ans);
}
int main(){
	int t=1;
	// scanf("%d",&t);
	while(t--) Solve();
	cerr<<clock()*1.0/CLOCKS_PER_SEC<<'\n';
}