Compare scan rows

A press operator compares two scans of the same calibration sheet, one taken before a maintenance stop and one after. Each scan is a grid of single character ink codes, one code per cell, and the two scans need not even agree on how many rows they have. A row matches when both scans have that row, the two rows hold the same number of cells, and every cell carries the same code. The scanner writes '?' for a cell it could not read, and a '?' on either side never counts as a difference. Return the index of the lowest numbered row that does not match. A row that only one scan has counts as a difference. Return -1 when every row matches. Codes are case sensitive.