import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;

class Main{

    public static void main(String[] args) {
        File file = new File("dezastru.in");
        double[][] mat = null;
        double[] p = null;
        int n = 0,k = 0;
        try {
            Scanner sc = new Scanner(file);
            if(sc.hasNextInt()) {
                n = sc.nextInt();
            }
            if(sc.hasNextInt()) {
                k = sc.nextInt();
            }
            p = new double[n];
            mat = new double[n][n];
            int i = 0;
            while(sc.hasNextDouble()) {
                p[i] = sc.nextDouble();
                i++;
            }
            sc.close();
        }
        catch(FileNotFoundException e) {
            e.printStackTrace();
        }

        mat[0][0] = p[0];
        for(int i = 1; i < n; i++) {
            mat[i][0] = mat[i-1][0] + p[i];
        }
        for(int i = 1; i < n; i++) {
            for(int j = 1; j < n; j++) {
                if(i != 0 && j != 0) {
                    if(j > i)
                        mat[i][j] = 0;
                    else {
                        mat[i][j] = mat[i-1][j] + mat[i-1][j-1] * p[i];
                    }
                }
            }
        }
        int prob = 1;
        for(int i = k+1; i <= n; i++)
            prob *= i;
        /*
        for(int i = 0; i < n; i++) {
            for(int j = 0; j < n; j++) {
                System.out.print(mat[i][j] + " ");
            }
            System.out.println("");
        }*/
        
        //System.out.println(mat[n-1][k-1] / prob);
        
        try {
            PrintWriter wr = new PrintWriter("dezastru.out");
            wr.println(mat[n-1][k-1] / prob);
            wr.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
        
    }
    
}