#include <bits/stdc++.h>
using namespace std;

using ll = long long;

#define rep(i, n) for (int i=0; i<(int)(n); ++(i))
#define rep3(i, m, n) for (int i=(m); (i)<(int)(n); ++(i))
#define repr(i, n) for (int i=(int)(n)-1; (i)>=0; --(i))
#define rep3r(i, m, n) for (int i=(int)(n)-1; (i)>=(int)(m); --(i))
#define all(x) (x).begin(), (x).end()

int main() {
	int n, m;
	cin >> n >> m;
	vector<int> b(m);
	rep(i, m) cin >> b[i];
	double mval = 0.0;
	ll sum = 0;
	repr(i, m) {
		sum += b[i];
		mval = max(mval, ((double)(sum)/(m-i))-(b.front()+((double)m/(m-i))));
	}
	printf("%.6f\n", (double)((n-1)-mval));
	return 0;
}