#include <bits/stdc++.h>
#define Dim 27
FILE *fin  = freopen("dezastru.in", "r", stdin);
FILE *fout = freopen("dezastru.out", "w", stdout);

using namespace std;
int n, k;
double A[Dim][Dim], p[Dim], C[Dim][Dim];
void read()
{
    scanf("%d %d", &n, &k);
    for(int i = 1; i <= n; ++ i)
        scanf("%lf", &p[i]);
}
void solve()
{
    int i, j;
    A[1][1] = p[1];
    A[1][0] = C[1][1] = C[1][0] = 1;
    for(i = 2; i <= n; ++ i)
    {
        A[i][0] = C[i][0] = 1;
        for(j = 1; j <= k && j <= i; ++ j)
        {
            A[i][j] = A[i - 1][j] + A[i - 1][j - 1] * p[i];
            C[i][j] = C[i - 1][j] + C[i - 1][j - 1];
        }
    }
    A[n][k] = A[n][k] / C[n][k];

}
void write()
{
    printf("%.7lf\n", A[n][k]);
}
int main()
{
    read();
    solve();
    write();
    return 0;
}