def combs(n)
  if n == 1 then
    (1..6).each {|x| yield [x] }
  else
    (1..6).each {|x| combs(n -1) {|arr| yield arr.unshift x ; arr.shift x  } }
  end
end

def prob(n, diff)
  total, hits = 6**n, 0
  combs(n) {|arr|  hits+= 1 if arr.inject(0, &:+) > diff  }
  hits.to_f/ total
end