#include <bits/stdc++.h>
using namespace std;
int n,k;
float b[26],s[26][26],d[26][26];
int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    for(int i=1;i<=n;i++)
    {
        scanf("%f",&b[i]);
        s[i][0]=d[i][0]=1;
    }
    s[0][0]=d[0][0]=1;
    for(int i=1;i<=n;i++)
    for(int j=1;j<=k;j++)
    {
        s[i][j]=s[i-1][j]+s[i-1][j-1]*b[i];
        d[i][j]=d[i-1][j]+d[i-1][j-1];
    }
    float sol=s[n][k]/d[n][k];
    printf("%.7f",sol);
    return 0;
}