Given a set of data, output the middle item (if even number of items, output the two middle items). If the input is 5 7 9 11 13 -1 (a negative indicates end), the output is 9. If the input is 5 7 9 11 -1, the output is 7 9. If the input is -1, the output is " " (a blank space).

Hint: First read the data into a vector. Then, based on the vector's size, find the middle item(s).