#include <iostream>
#include <fstream>
#include <string>
#include <algorithm>
#include <vector>
#include <map>
#include <queue>

#define LL long long
#define ULL unsigned long long
#define PB push_back

using namespace std;

double A[26][26];
double B[26];

int main(void){

    int i,j,N,k,M;
    
    ifstream in("dezastru.in");
    ofstream out("dezastru.out");
    
    in >> N >> M;
    
    for (i=1;i<=N;i++)
        in >> B[i];
    
    A[0][0] = 1;
    
    for(i=1;i<=M;i++)
       for (j=1;j<=N;j++){
           for (k=j-1;k>=0;k--)
               A[i][j] += A[i-1][k];
           A[i][j] *= B[j];
           //out << A[i][j]<< " " << i << " " << j << "\n";
       }
    
    double sol = 0;
    for(i=1;i<=N;i++)    
       sol += A[M][i];   

    for(i=2;i<=(N-M);i++)
       sol *= i;           
    for(i=M+1;i<=N;i++)
       sol /= i;
    out.precision(6);
    out << sol;
    
    in.close();
    out.close();

return 0;
}