#include <bits/stdc++.h>

#include "testlib.h"
using namespace std;

int n, m;
long double res = 1e18;
long double cost;
long long base;
int main() {
	registerValidation();
	int n = inf.readInt(1, 1000000000);
	inf.readSpace();
	int m = inf.readInt(1, min(n, 100000));
	inf.readEoln();
	vector<int> b(m);
	int bef = 0;
	for(int i = 0; i < m; i++) {
		if(i) inf.readSpace();
		b[i] = inf.readInt(bef + 1, n);
		bef = b[i];
	}
	inf.readEoln();
	inf.readEof();
	if(m <= 1) {
		printf("%d\n", n - 1);
		return 0;
	}
	base += b.front() - 1 + n - b.back();
	res = b.back() - b.front();
	for(int i = m - 1; i >= 1; i--) {
		cost += b.back() - b[i];
		res = min(res, cost / (m - i) + (long double)m / (m - i));
	}
	cout << fixed << setprecision(6);
	cout << res + base << endl;
}