import java.io.BufferedReader;
import java.io.InputStreamReader;

public class Main {
	public static void main(String[] args) throws Exception {
		BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
		String[] sa = br.readLine().split(" ");
		int n = Integer.parseInt(sa[0]);
		int m = Integer.parseInt(sa[1]);

		sa = br.readLine().split(" ");
		int[] b = new int[m];
		for (int i = 0; i < m; i++) {
			b[i] = Integer.parseInt(sa[i]);
		}
		br.close();

		long sum = 0;
		int cnt = 0;
		for (int i = 0; i < m; i++) {
			int d = b[m - 1] - b[i];
			if (d <= m) {
				sum += d;
				cnt++;
			}
		}
		double e1 = (double) m / cnt;
		double e2 = (double) sum / cnt;
		double e = Math.min(b[m - 1] - b[0], e1 + e2);
		double ans = b[0] - 1 + e + n - b[m - 1];
		System.out.println(ans);
	}
}