#include<bits/stdc++.h>

using namespace std;

double ans,dv,a[200],A[30][30];

int main()
{
    int n,k,i,j;
    ifstream cin("dezastru.in");
    ofstream cout("dezastru.out");
    cin>>n>>k;
    for (i=1;i<=n;i++)
        cin>>a[i],A[i][0]=1;
    A[0][0]=1;
    for (i=1;i<=n;i++)
        for (j=1;j<=i;j++)
            A[i][j]=A[i-1][j-1]*a[i]+A[i-1][j];
    double d=A[n][k],f=1;
    for (i=k+1;i<=n;i++)
        f*=i;
    for (i=2;i<=n-k;i++)
        f/=i;
    cout<<setprecision(6)<<d/f;
}