#include <iomanip>
#include <iostream>
using namespace std;
int n, m, p;
int main() {
	while (cin >> n >> m, n) {
		int f = 0;
		for (int i = 0; i < m; i++) {
			cin >> p;
			if (p == 1) f = 1;
		}
		long double r = f ? 0 : n / 2.0;
		cout << fixed << setprecision(15) << r << endl;
	}
}