#include <cstdio>
#define MAX_N 26
using namespace std;
int N, K;
float P[MAX_N], v[MAX_N][MAX_N];
FILE *f, *g;

int main()
{
    f = fopen("dezastru.in", "r");
    g = fopen("dezastru.out", "w");

    fscanf(f, "%d%d", &N, &K);
    for(int i=0; i<N; i++)
        fscanf(f, "%f", &P[i]);

    for(int i=0; i<N; i++)
        v[i][0]=1;

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

        fprintf(g, "%f ", v[N][K]);
        fprintf(g, "\n");


    fclose(f);
    fclose(g);
    return 0;
}