/*
    If you can't explain it simply, you don't understand it well enough.
*/

#include <cstdio>
#include <algorithm>

using namespace std;

const int Nmax = 30;

int n , k , i , j;
double D[Nmax][Nmax] , p[Nmax];

int combinari(int n , int k)
{
    int ans = 1;
    
    for (i  = k + 1; i <= n; ++i)
        ans *= i;
    for (i = 1; i <= n - k; ++i)
        ans /= i;
    
    return ans;
}

int main()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    
    scanf("%d %d", &n, &k);
    
    for (i = 1; i <= n; ++i)
        scanf("%lf", &p[i]);
    
    for (i = 0; i <= n; ++i)
        D[i][0] = 1;
    
    for (i = 1; i <= n; ++i)
        for (j = 1; j <= i; ++j)
            D[i][j] = D[i-1][j] + D[i-1][j-1] * p[i];
    
    printf("%.6lf", D[n][k] / combinari(n , k));
    
    return 0;
}