#include <stdio.h>

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

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