#include <stdio.h>
#include <stdlib.h>
double comb[26][26],suma[26][26],v[26];
int main()
{
    int n,k,i,j;
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    for(i=1; i<=n; i++)
        scanf("%lf",&v[i]);
    for(i=0; i<=n; i++)
        suma[i][0]=comb[i][0]=1;
    for(i=1; i<=n; i++)
        for(j=1; j<=k; j++)
        {
            suma[i][j]=suma[i-1][j]+suma[i-1][j-1]*v[i];
            comb[i][j]=comb[i-1][j]+comb[i-1][j-1];
        }
    printf("%.6lf",suma[n][k]/comb[n][k]);

    return 0;
}