#include<stdio.h>

float a[27],b[27][27],s;
long long nr,i,j,n,k;


int main()
{ 
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    
    scanf("%d %d",&n,&k);
    
    for(i=1;i<=n;i++) scanf("%f",&a[i]);
    for(i=0;i<=n;i++) b[i][0]=1;
    for(i=1;i<=n;i++) 
        for(j=1;j<=i;j++) b[i][j]=b[i-1][j]+b[i-1][j-1]*a[i];
   nr=1;
   j=1;      
   for(i=k+1;i<=n;i++) nr*=i;
   for(i=1;i<=n-k;i++) j*=i;
   nr/=j;
                      
   printf("%f\n",b[n][k]/nr);     
    
    
    fclose(stdin);
    fclose(stdout);
    return 0;
}