#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

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

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

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

long long fact(int n)
{
    long long rez = 1;

    for ( int i = 2; i <= n; ++i )
        rez *= i;

    return rez;
}

long long cnk(int n, int k)
{
    return fact(n)/(fact(n-k)*fact(k));
}

int main()
{
    read_init();

    for ( int i = 2; i <= n; ++i )
        for ( int j = 2; j <= k; ++j )
            a[i][j] = a[i-1][j] + a[i-1][j-1]*nr[i];

    fprintf(out, "%.6lf\n", a[n][k]/cnk(n,k));

	return 0;
}