The last expression is negative. The first and second are positive, because there are an even amount of negative digits. The third expression equals to zero, which is neither negative or positive. The last expression is negative because there is an uneven number of negative digits. It's the same reading why any negative number to the power of 2,4,6,8,..., is positive and any negative number to the power of 1,3,5,7,9,..., is negative