A Unique Unbiased Estimator with an Interesting Property

Suppose an unknown proportion p of the elements of an infinite population have size a > O, while the remainder all have size b > a. We draw a random sample X of size three,(x1,x2,x3), from the infinite population. We then draw from X a sample Y of two elements, successively without replacement and with probability proportional to size (ppswor sampling). Then it turns out that when only Y is observable, there is a unique unbiased estimator of p, which has the interesting property of being a RUBE (ridiculous unbiased estimator), since it takes values outside of (O,1).