#include<stdio.h>   
int c[30][30],n,k;   
float v[30],a[30][30];
void dinamica()   
{   
    int i,j;   
    c[0][1]=1;   
    a[0][1]=1.0;   
    k++;   
    for(i=1;i<=n;++i)   
        for(j=1;j<=k;++j){   
            c[i][j]=c[i-1][j]+c[i-1][j-1];   
            a[i][j]=a[i-1][j]+a[i-1][j-1]*v[i];   
        }
	printf("%.6f",(float)a[n][k]/c[n][k]);   
}   
int main()   
{   
    int i;   
    freopen("dezastru.in","r",stdin);   
    freopen("dezastru.out","w",stdout);   
    scanf("%d %d",&n,&k);   
    for(i=1;i<=n;++i)   
        scanf("%f",&v[i]);   
    dinamica();
    return 0; 
}