#include <cstdio>

#define maxn 26

FILE *in = fopen("dezastru.in","r"), *out = fopen("dezastru.out","w");

int n, k;
double nr[maxn];
double a[maxn][maxn]; // a[i][j] == suma tuturor produselor de j factori alesi din primele i numere
long long c[maxn][maxn];

void read_init()
{
    fscanf(in, "%d %d", &n, &k);

    for ( int i = 1; i <= n; ++i )
        fscanf(in, "%lf", &nr[i]);

    for ( int i = 0; i <= n; ++i )
        a[i][0] = 1;
}

void cnk()
{
    for ( int i = 1; i <= n; ++i )
        c[i][1] = i;
    for ( int i = 1; i <= n; ++i )
        for ( int j = 2; j <= k; ++j )
            c[i][j] = c[i-1][j] + c[i-1][j-1];
}

int main()
{
    read_init();
    cnk();

    for ( int i = 1; i <= n; ++i )
        for ( int j = 1; j <= k; ++j )
            a[i][j] = a[i-1][j] + a[i-1][j-1]*nr[i];

//    for ( int i = 0; i < n; ++i )
//    {
//        for ( int j = 0; j < k; ++j )
//            printf("%lf ", a[i][j]);
//        printf("\n");
//    }

    fprintf(out, "%.6lf\n", (double)a[n][k]/c[n][k]);
	return 0;
}