#include<cstdio>
#define DIM 26
using namespace std;

int N, K;
float v[DIM], d[DIM][DIM], comb[DIM][DIM];

int main()
    {
        freopen("dezastru.in","r",stdin);
        freopen("dezastru.out","w",stdout);

        scanf("%d%d", &N, &K);
        for (int i = 1; i <= N; i++)
        {
            scanf("%f", &v[i]);
            d[i][0] = 1;
            comb[i][1] = i;
        }

        d[0][0] = 1;

        for (int i = 1; i <= K; i++)
            for (int j = i; j <= N; j++)
                d[j][i] = d[j - 1][i] + v[j] * d[j - 1][i - 1];

        for (int i = 2; i <= N; i++)
            for (int j = 2; j <= i; j++)
                comb[i][j] = comb[i - 1][j] + comb[i - 1][j - 1];

        printf("%.6f", d[N][K] / comb[N][K]);
    }