// AOJ 1076 Time Manipulation
// 2018.2.3 bal4u

#include <stdio.h>

//#define getchar_unlocked()  getchar()
int in()
{
	int n = 0;
	int c = getchar_unlocked();
	do n = (n<<3)+(n<<1) + (c & 0xf), c = getchar_unlocked();
	while (c >= '0');
	return n;
}

int main()
{
	int n, m, f;

	while (n = in()) {
		m = in();

		f = 0;
		while (m--) {
			if (in() == 1) f = 1;
		}

		if (f) puts("0.0000000000");
		else printf("%.10lf\n", n/2.0);
	}
	return 0;
}
