#include <stdio.h>
#define NMax 30

int n, k;
double p[NMax][NMax], pos[NMax][NMax];

void citire();
void rez();

int main()
{
	citire();
	rez();
	return 0;
}

void rez()
{
	int l, x, y, suma = 0;
	double prod = 0;

	for (l=2; l<=k; l++)
	{
		// siruri de lungime l din l - 1
		for (x=1; x<=n; x++)
		{
			// pun pe ultima pozitie nr X
			suma = 0;
 			for (y=1; y<x; y++)
			{
				if (pos[y][l-1])
				{
					p[x][l]   += (p[y][l-1] * pos[y][l-1]);
					suma += pos[y][l-1];
					pos[x][l] += pos[y][l-1];
				}
			}

			if ( suma != 0 )
				p[x][l] = p[x][l] * p[x][1] / suma;
			else
				p[x][l] = 0;
				//p[x][l] = p[x][l] * p[x][1] / (x - 1);
		}
	}
	
	suma = 0;
	for (x=1; x<=n; x++)
	{
		suma += pos[x][k];
		prod += p[x][k] * pos[x][k];
	}

	printf( "%.6lf\n", prod / suma );
}
void citire()
{	
	int i;

	freopen( "dezastru.in", "rt", stdin );
	freopen( "dezastru.out", "wt", stdout );

	scanf( "%d %d", &n, &k );

	for (i=1; i<=n; i++)
	{
		scanf( "%lf", &p[i][1] );
		pos[i][1] = 1;
	}
}