#include <cstdio>
#include <algorithm>

int main(){
	int n, k;
	FILE* fin = fopen("save.txt", "rt");
	fin = stdin;
	
	fscanf(fin, "%d %d\n", &n, &k);
	
	long double times[k];
	
	for(long long i = 0; i < k; i++){
		fscanf(fin, "%Lf", &times[i]);
	}
	
	std::sort(times, times + k);
	
	long long d1;
	long long i = 0; long long j;
	long double dif; long double a;
	long long d = 1;
	i = k - n;
	
	j = i-1;
	while(j >= 0 && times[j] == times[i]){
		d++;
		j--;
		if(i > 0) i--;
	}
	j = k - n + 1;
	while(j < k && times[j] == times[i]){
		d++;
		j++;
	}
	long double total = 0;
	while(times[k - n] >= 0.0000000001){
		if(i > 0){
			dif = times[i] - times[i - 1];
			j = i - 1;
			while(j >= 0 && times[j] == times[i]){
				d++;
				j--;
				if(i > 0) i--;
			}
		}
		else{
			dif = times[0];
			i = 0;
		}
		
		d1 = i + d - k + n;

		if(d > d1 && d < k - 1) a = (times[i+d] - times[i])*d1/(d-d1);
		else a = dif+1;

		if(!(a < dif)) a = dif;
		total += a*d/d1;
		for(j = i; j <= i+d-1; j++){
			times[j] -= a;
		}
		for(j = i+d; j < k; j++){
			times[j] -= a*d/d1;
		}
		
		j = i+d;
		while(j < k && times[j] == times[i]){
			d++;
			j++;
		}
	}

	printf("%.9Lf\n", total);
	
	return 0;
}