HZNUOJ

Practice

Tags:
Time Limit:  1 s      Memory Limit:   128 MB
Submission:1     AC:1     Score:99.94

Description

How much does winning ACM depend on practice?

We assume that p, the probability that a given team will win a given contest, is related to n, the number of practice problems solved by the team prior to the contest. This relationship is modelled by the logistic formula

   log(p/(1-p)) = a + b n ,

for some

a

and

b

. Your job is to find

a

and

b

such that the formula most accurately reflects a set of observed results.

Each observation consists of n and w. n is the number of practice problems solved by some team prior to a contest, and w is 1 if the team wins the contest, 0 if it does not.

Given a, b, and n the formula above may be used to compute p, the estimated probability that w = 1. The likelihood of a particular observation is p if w = 1 and 1-p if w = 0; The likelihood of a set of observations is the product of the likelihoods of the individual observations.

You are to compute the maximum likelihood estimate for a and b. That is, the values of a and b for which the likelihood of a given set of observations is maximized.

The input contains several test cases followed by a line contatining 0. Each test case begins with 1 < k ≤ 100, the number of observations that follow. Each observation consists of integers 0 ≤ n ≤ 100 and 0 ≤ w ≤ 1. The input will contain at least two distinct values of n and of w. For each test case, output a single line containing a and b, rounded to four digits to the right of the decimal.

Samples

input
20 0 0 0 0 0 0 0 0 1 0 1 0 1 0 1 1 2 0 2 0 2 1 2 1 3 0 3 1 3 1 3 1 4 1 4 1 4 1 4 1 0
output
-3.1748 1.5874