#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define P pair<ll,ll>
#define FOR(I,A,B) for(ll I = ll(A); I < ll(B); ++I)
#define FORR(I,A,B) for(ll I = ll((B)-1); I >= ll(A); --I)
#define TO(x,t,f) ((x)?(t):(f))
#define SORT(x) (sort(x.begin(),x.end())) // 0 2 2 3 4 5 8 9
#define POSL(x,v) (lower_bound(x.begin(),x.end(),v)-x.begin()) //xi>=v  x is sorted
#define POSU(x,v) (upper_bound(x.begin(),x.end(),v)-x.begin()) //xi>v  x is sorted
#define NUM(x,v) (POSU(x,v)-POSL(x,v))  //x is sorted
#define REV(x) (reverse(x.begin(),x.end())) //reverse
ll gcd_(ll a,ll b){if(a%b==0)return b;return gcd_(b,a%b);}
ll lcm_(ll a,ll b){ll c=gcd_(a,b);return ((a/c)*b);}
#define NEXTP(x) next_permutation(x.begin(),x.end())
const ll INF=ll(1e16)+ll(7);
const ll MOD=1000000007LL;
#define out(a) cout<<fixed<<setprecision((a))
//tie(a,b,c) = make_tuple(10,9,87);
#define pop_(a) __builtin_popcount((a))
ll keta(ll a){ll r=0;while(a){a/=10;r++;}return r;}
#define num_l(a,v) POSL(a,v) //v未満の要素数
#define num_eql(a,v) POSU(a,v) //v以下の要素数
#define num_h(a,v) (a.size() - POSU(a,v)) //vより大きい要素数
#define num_eqh(a,v) (a.size() - POSL(a,v)) //v以上の要素数
// static_cast< long long ll >



int main(){
	ll N,M;
	cin >> N >> M;
	vector<int> B(M);
	FOR(i,0,M) cin >> B[i];
	vector<ll> sum(M+1,0);
	FORR(i,0,M){
		sum[i] = sum[i+1] + N - B[i];
	}
	long double ans = N-1;
	FOR(i,0,M){ // iまでワープする
		int num = i+1;
		long double S = sum[i+1];
		long double t = (S/M + 1.0*num/M) / (1.0-1.0*num/M) + 1.0*B[0];
		ans = min(ans,t);
	}
	out(6) << ans << endl;
}