#include<stdio.h>

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


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];
    for(i=1;i<=n;i++) 
    { c[i][1]=i;
      for(j=2;j<=k;j++) c[i][j]=c[i-1][j]+c[i-1][j-1];
    }  
                      
   printf("%f\n",b[n][k]/c[n][k]);     
    
    
    fclose(stdin);
    fclose(stdout);
    return 0;
}