n,k=gets.split.map &:to_i
a=gets.split.map &:to_r
x=[[0r,0r]]
1.upto(n-2){|i|
  s=a[i-1]+a[i+1]
  l=a[i-1]/s
  b=1-l*x[-1][1]
  x<<[l*x[-1][0]/b,a[i+1]/s/b]
}
y=[]
y[n-1]=1r
(n-2).downto(0){|i|y[i]=x[i][0]+x[i][1]*y[i+1]}
ans=y[k-1]
puts ans==0r ? 0 : ans