#include <iostream>
#include <cstdio>
#include <algorithm>
#define rep(i, n) for(i = 0; i < n; i++)
#define int long long
using namespace std;

int n, m;
int b[100000];

signed main() {
	int i, j;
	
	cin >> n >> m;
	rep(i, m) cin >> b[i];
	sort(b, b + m);
	
	double ans = n - 1;
	int sumGoal = 0;
	for (i = m - 1; i >= 1; i--) {
		sumGoal += n - b[i];
		double expGoal = (double)sumGoal / (m - i);
		
		double cst = b[0] - 1 + (double)m / (m - i) + expGoal;
		ans = min(ans, cst);
	}
	
	printf("%.14f\n", ans);
	return 0;
}