Worksheet range sum

A finance team exports a worksheet as a rectangular grid of whole numbers, where sheet[r][c] holds the value in row r and column c counting from zero. Ranges arrive as text in worksheet notation, for example B2:C3. A reference is one uppercase letter from A to Z naming the column, where A is column 0, followed by a row number counting from 1, so B2 means sheet[1][1]. The two references name opposite corners of a rectangle, but they can arrive in either order and either corner can come first, so C3:A1 and A3:C1 cover the same block as A1:C3. Return the sum of every value inside that rectangle, both corners included. Every reference lands inside the sheet.