Given an array of prices [p1,p2...,pn] and a target, round each price pi to Roundi(pi) so that the rounded array [Round1(p1),Round2(p2)...,Roundn(pn)] sums to the given target. Each operation Roundi(pi) could be either Floor(pi) or Ceil(pi). Return the string "-1" if the rounded array is impossible to sum to target. Otherwise, return the smallest rounding error, which is defined as Σ |Roundi(pi) - (pi)| for i from 1 to n, as a string with three places after the decimal. Example 1: Input: prices = ["0.700","2.800","4.900"], target = 8 Output: "1.000" Explanation: Use Floor, Ceil and Ceil operations to get (0.7 - 0) + (3 - 2.8) + (5 - 4.9) = 0.7 + 0.2 + 0.1 = 1.0 . Example 2: Input: prices = ["1.500","2.500","3.500"], target = 10 Output: "-1" Explanation: It is impossible to meet the target. Example 3: Input: prices = ["1.500","2.500","3.500"], target = 9 Output: "1.500" Constraints: 1 <= prices.length <= 500 Each string prices[i] represents a real number in the range [0.0, 1000.0] and has exactly 3 decimal places. 0 <= target <= 106