#include <stdio.h>

int main()
{
	int i, N, M, B[100002];
	scanf("%d %d", &N, &M);
	for (i = 1; i <= M; i++) scanf("%d", &(B[i]));

	long double dp[100001], sum;
	for (i = M - 1, dp[M] = (long double)(N - B[M]), sum = dp[M]; i >= 1; i--) {
		if ((sum + M) / (M - i) < (long double)(N - B[i])) break;
		dp[i] = (long double)(N - B[i]);
		sum += dp[i];
	}
	for (dp[i] = (sum + M) / (M - i), i--; i >= 1; i--) dp[i] = dp[i+1];
	printf("%.5Lf\n", dp[1] + B[1] - 1);
	fflush(stdout);
	return 0;
}