#include <stdio.h>

char n,k,i,j;
float p,b[30],m[30][30];
int c=1;

int main () {
    freopen ("dezastru.in","r",stdin);
    freopen ("dezastru.out","w",stdout);
    scanf ("%hd%hd",&n,&k);
    for (i=k+1; i<=n; i++) c*=i;
    for (i=1; i<=n-k; i++) c/=i;
    for (i=1; i<=n; i++) scanf ("%f",b+i);
    for (i=0; i<=n; i++) m[0][i]=1;
    for (i=1; i<=k; i++)
		for (j=i; j<=n; j++)
			m[i][j]=m[i][j-1]+b[j]*m[i-1][j-1];
	printf ("%.6f\n",m[k][n]/c);
    return 0;
}