#include<stdio.h>

double sol,d[36][36],v[36];
int n,k;

int main ()
{
    int i,j;
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    for(i=1;i<=n;i++)
        scanf("%lf",&v[i]);
    for(i=0;i<=n;i++)
        d[i][0]=1;
    for(i=1;i<=n;i++)
        for(j=1;j<=k;j++)
            d[i][j]=d[i-1][j]+d[i-1][j-1]*v[i];
    sol=d[n][k];
    for(i=k+1;i<=n;i++)
        sol/=i;
    for(i=2;i<=n-k;i++)
        sol*=i;
    printf("%.6lf",sol);
    return 0;
}