import std.stdio;
import std.c.stdio;
import std.range;
import std.array;
import std.functional;
import std.algorithm;
import std.conv;
import std.container;
import std.math;
import std.numeric;
import std.string;
import std.c.string;
import std.regex;
import std.typecons;
 
void main() {
    ulong N, M;
    int[] p;
    bool input() {
        scanf("%lld %lld\n", &N, &M);
        if (N == 0 && M == 0) return false;
        p = new int[M];
        foreach (i; 0 .. M) {
            scanf("%d", &p[i]);
        }
        scanf("\n");
        p.sort;
        return true;
    }

    void solve() {
        writefln("%.8f", (p[0] == 1 ? 0.0 : N / 2.0));
    }

    while (input) solve;
}