#include <stdio.h>
#include <bits/stdc++.h>
#include <atcoder/all>
using namespace atcoder;
using mint = modint998244353;
using namespace std;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf 1000000001

int main(){
	
	int N,M;
	cin>>N>>M;
	
	vector<int> B(M);
	rep(i,M){
		cin>>B[i];
	}
	
	double ans = B[0]-1.0;
	
	vector<double> dp(M);
	dp.back() = N-B.back();
	double sum = dp.back();
	for(int i=M-2;i>=0;i--){
		double E = M;
		E /= M-1-i;
		E += sum / (((double)M-i-1));
		dp[i] = min(E,(double)N-B[i]);
		sum += dp[i];
	}
	/*
	rep(i,dp.size()){
		cout<<dp[i]<<',';
	}
	cout<<endl;
	*/
	ans += dp[0];
	
	cout<<fixed<<setprecision(10)<<ans<<endl;
	
    return 0;
}