#pragma GCC optimize("Ofast")
#define _GLIBCXX_DEBUG
#include<bits/stdc++.h>
using namespace::std;
struct __INIT{__INIT(){cin.tie(0);ios::sync_with_stdio(false);cout<<fixed<<setprecision(15);}} __init;
// random_device rd;
// mt19937 mt(rd());
// #include <boost/numeric/interval.hpp>
// #include <boost/numeric/interval/io.hpp>
// #include <boost/intrusive/rbtree.hpp>
// #include <boost/geometry.hpp>
// #include <boost/geometry/geometries/linestring.hpp>
// #include <boost/geometry/geometries/polygon.hpp>
// #include <boost/geometry/geometries/point_xy.hpp>
// #include <boost/multiprecision/cpp_dec_float.hpp>
// #include <boost/multiprecision/cpp_int.hpp>
// namespace mp = boost::multiprecision;
// typedef mp::number<mp::cpp_dec_float<0>> cfloat;
// typedef mp::cpp_int cint;

#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#include <ext/pb_ds/priority_queue.hpp>
#include<ext/pb_ds/tag_and_trait.hpp>
using namespace __gnu_pbds;
template<typename T=long long>using pbds=tree<T,null_type,less<T>,rb_tree_tag,tree_order_statistics_node_update>;
template<typename T=long long>using pqueue =__gnu_pbds::priority_queue<T, less<T>,rc_binomial_heap_tag>;
typedef long long lint;
typedef long long ll;
typedef long double ldouble;
typedef vector<lint> vec;
typedef vector<vector<lint>> mat;
typedef vector<vector<vector<lint>>> mat3;
typedef vector<double> dvec;
typedef vector<vector<double>> dmat;
typedef vector<vector<vector<double>>> dmat3;
typedef vector<string> svec;
typedef vector<vector<string>> smat;
typedef vector<vector<vector<string>>> smat3;
typedef vector<pair<lint,lint>> pvec;
typedef vector<vector<pair<lint,lint>>> pmat;
typedef vector<vector<vector<pair<lint,lint>>>> pmat3;
#define rep(i, n) for(lint i = 0; i < (lint)(n); i++)
#define irep(i) for(lint i = 0;; i++)
#define irep1(i) for(lint i = 1;; i++)
#define irep2(i) for(lint i = 2;; i++)
#define rrep(i, n) for(lint i = (lint)(n-1); i >-1; i--)
#define rrepi(i,a,b) for(lint i = (lint)(b-1); i >a-1; i--)
#define repi(i,a,b) for(lint i=lint(a);i<lint(b);i++)
#define rep2(i,a,b,c) for(lint i=lint(a);i>lint(b);i+=c)
#define all(x) (x).begin(),(x).end()
//#define PI 3.141592653589793
#define dist(x1,y1,x2,y2) (pow(pow(x2-x1,2)+pow(y2-y1,2),0.5))
#define output(v) do{bool f=0;for(auto i:v){cout<<(f?" ":"");if(i>INF/2)cout<<"INF";else cout<<i;f=1;}cout<<"\n";}while(0)
#define output2(v) for(auto j:v)output(j);
#define _output(v) do{bool f=0;rep(i,v.size()){cout<<(f?" ":"");if(v[i]>INF/2)cout<<"INF";else cout<<v[i];f=1;}cout<<"\n";}while(0)
#define _output2(v) rep(j,v.size()){_output(v[j]);}
#define input(a,n) lint n;cin>>n;vector<lint>a(n);rep(i,n)cin>>a[i];
#define SUM(v) accumulate(all(v),0LL)
#define INF (1LL<<60)
#define IINF (1<<30)
#define EPS (1e-10)
#define LINF 9223372036854775807
#define MOD 998244353
#define endl "\n"
template<typename T=lint>T in(){return *istream_iterator<T>(cin);}
inline lint gcd(lint a,lint b){return b?gcd(b,a%b):a;}
template<typename T=lint>inline T lcm(T a,T b){return a/gcd(a,b)*b;}
template<typename T=lint>inline T add(T a,T b){return a+b;}
template<typename T=lint>inline T minq(T a,T b){return min(a,b);}
inline bool chmin(auto& s,const auto& t){bool res=s>t;s=min(s,t);return res;}
inline bool chmax(auto& s,const auto& t){bool res=s<t;s=max(s,t);return res;}
vector<lint> dx={-1,1,0,0,1,1,-1,-1};
vector<lint> dy={0,0,-1,1,1,-1,1,-1};



int main(){
    while(1){
        double n;
        lint m;
        cin>>n>>m;
        if(n==0&&m==0)break;
        vec a(m);
        rep(i,m)cin>>a[i];
        double s=n*(n+1)/2,t=n;
        rep(i,1LL<<m){
            lint cnt=0,val=0;
            rep(j,m){
                if(i&1<<j){
                    cnt++;
                    if(val==0)val=a[j];
                    else val=lcm(val,a[j]);
                }
            }
            if(cnt==0)continue;
            if(cnt%2)t-=n/val;
            else t+=n/val;
            if(cnt%2)s-=val*(n/val)*(n/val+1)/2;
            else s+=val*(n/val)*(n/val+1)/2;
        }
        if(t==0)cout<<0<<endl;
        else cout<<s/t<<endl;
    }
}


Measured memory_bytes: 3260