#include "bits/stdc++.h"
//#include "atcoder/all"
using namespace std;
//using namespace atcoder;
//using mint = modint1000000007;
//const int mod = 1000000007;
//using mint = modint998244353;
//const int mod = 998244353;
const int INF = 1e9;
//const long long LINF = 1e18;
#define rep(i, n) for (int i = 0; i < (n); ++i)
#define rep2(i,l,r)for(int i=(l);i<(r);++i)
#define rrep(i, n) for (int i = (n-1); i >= 0; --i)
#define rrep2(i,l,r)for(int i=(r-1);i>=(l);--i)
#define all(x) (x).begin(),(x).end()
#define allR(x) (x).rbegin(),(x).rend()
#define endl "\n"
int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	int n, m;
	cin >> n >> m;
	vector<int>b(m);
	rep(i, m) { cin >> b[i]; b[i]--; }
	double ans = n - 1;
	vector<double>sum(m + 1);
	sum[0] = 0;
	rep(i, m) { sum[i + 1] = sum[i] + (n - 1) - b[(m - 1) - i]; }

	rep2(i, 1, m) {
		//i個目までのワープパネルには乗り続ける。
		double anssub = (double)b[0] + m / ((double)m - i) + sum[m - i] / (m - i);
		ans = min(anssub, ans);
	}
	printf("%.16f\n", ans);
	return 0;
}