#include <fstream>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
double comb[105][105], v[105], s;
int n, m, i, j;
int main()
{
    f >> n >> m;
    for(i = 1; i <= n; ++ i)
        f >> v[i];
    for(i = 0; i <= n; ++ i)
        comb[i][0] = 1;
    for(i = 1; i <= n; ++ i)
        for(j = 1; j <= m; ++ j)
         comb[i][j] = comb[i - 1][j] + v[i] * comb[i - 1][j - 1];
    s = comb[n][m];
    for(i = 1; i <= m; ++ i)
    {
        s /= (n - m + i);
        s = s * i;
    }
    g << s;
    return 0;
}