#include<stdio.h>
int n,k,i,j;
double p[26],a[26][26],c[26][26];
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",p+i),a[i][0]=1,c[i][1]=i;
    for(a[0][0]=i=1;i<=k;++i)
        for(j=i;j<=n;++j)
            a[j][i]=a[j-1][i]+a[j-1][i-1]*p[j];
    for(i=2;i<=n;++i)
        for(j=2;j<=i;++j)
            c[i][j]=c[i-1][j-1]+c[i-1][j];
    printf("%.6lf",a[n][k]/c[n][k]);
    return 0;
}