#include <iostream>

using namespace std;

double s[30][30],a[30];
int n,k,i,j,c[30][30];

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

    scanf("%d%d", &n, &k);

    for(i=1; i<=n; ++i) scanf("%lf", &a[i]);

    c[0][0]=1;s[0][0]=1;

    for(i=1; i<=n; ++i)
    for(j=0; j<=k; ++j)
    {
        c[i][j]= c[i-1][j]+ c[i-1][j-1];
        s[i][j]= s[i-1][j]+ s[i-1][j-1]*a[i];
    }

    printf("%.6lf\n", s[n][k]/c[n][k]);

    return 0;
}