#include <bits/stdc++.h>
#define fst first
#define snd second
#define fore(i,a,b) for(int i=a,ThxDem=b;i<ThxDem;++i)
#define pb push_back
#define ALL(s) s.begin(),s.end()
#define FIN ios_base::sync_with_stdio(0);cin.tie(0);cout.tie(0)
#define SZ(s) int(s.size())
using namespace std;
typedef long long ll;
typedef pair<int,int> ii;

const int MAXN=11;
int m,k,n,a[MAXN],pt[MAXN];

ll cmb(ll n, ll k){
	ll ans=1;
	fore(i,n-k+1,n+1) ans*=i;
	fore(i,1,k+1) ans/=i;
	return ans;
}

ll fac(ll n){
	ll ans=1;
	fore(i,1,n+1) ans*=i;
	return ans;
}

double getval(vector<ll> extra){
	vector<ll> up=extra,dw;
	
	fore(i,0,m){
		fore(x,a[i]-pt[i]+1,a[i]+1) up.pb(x);
		fore(x,1,pt[i]+1) dw.pb(x);
	}
	
	fore(i,n-k+1,n+1) dw.pb(i);
	fore(i,1,k+1) up.pb(i);
	
	sort(ALL(up));
	sort(ALL(dw));
	double ans=1;
	
	while(SZ(up) || SZ(dw)){
		if(ans<=1){
			if(SZ(dw)) ans/=dw.back(),dw.pop_back();
			else ans*=up.back(),up.pop_back();
		}
		else{
			if(SZ(up)) ans*=up.back(),up.pop_back();
			else ans/=dw.back(),dw.pop_back();
		}
	}
	
	return ans;
}

// lef = cantidad de formas de elegir k elementos (sin importar orden)
double getlef(){
	return getval({});
}

// rig = permutaciones de k-1 elementos
double getrig(){
	vector<ll> extra;
	fore(i,1,k) extra.pb(i);
	
	double ans=0;
	fore(i,0,m) if(pt[i]){
		pt[i]--;
		ans+=getval(extra);
		pt[i]++;
	}
	
	return ans;
}

double go(int pos, int want){
	if(pos==m){
		return want==0?min(getlef(),getrig()):0.;
	}
	int top=min(want,a[pos]);
	double r=0;
	fore(i,0,top+1){
		pt[pos]=i;
		r+=go(pos+1,want-i);
	}
	return r;
}

int main(){FIN;
	cin>>k>>m;
	fore(i,0,m) cin>>a[i],n+=a[i];
	ll ans=go(0,k);
	cout<<fixed<<setprecision(10)<<go(0,k)<<"\n";
}