//Vasilut
#include<cstdio>
#define NN 26

using namespace std;


float bst[NN][NN],sir[NN],comb;
int n,k;



int main()
{

    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    int i,j;
    for(i=1;i<=n;i++)
    {
        scanf("%f",&sir[i]);
        bst[i][0]=1;
    }
    bst[0][0]=1;
    for(i=1;i<=n;i++)
    {
            for(j=1;j<=k;j++)
        {
        bst[i][j]=bst[i-1][j] + bst[i-1][j-1]*sir[i];
        }
    }
     comb=1;
    for(i=k+1;i<=n;i++)
    comb=comb*i;
    for(i=i;i<=n-k;i++)
    comb=comb/i;
    printf("%f",bst[n][k]/comb);

    return 0;
}