Annoying painting tool#3833
Maybe you wonder what an annoying painting tool is? First of all, the painting tool we speak of supports only black and white. Therefore, a picture consists of a rectangular area of pixels, which are either black or white. Second, there is only one operation how to change the colour of pixels:
Select a rectangular area of rows and columns of pixels, which is completely inside the picture. As a result of the operation, each pixel inside the selected rectangle changes its colour (from black to white, or from white to black).
Initially, all pixels are white. To create a picture, the operation described above can be applied several times. Can you paint a certain picture which you have in mind?
Contains several test cases. Each test case starts with one line containing four integers , , and (1 \le r \le n \le 100, 1 \le c \le m \le 100). The following lines each describe one row of pixels of the painting you want to create. The -th line consists of characters describing the desired pixel values of the -th row in the finished painting ('' indicates white, '' indicates black).
The last test case is followed by a line containing four zeros.
For each test case, print the minimum number of operations needed to create the painting, or if it is impossible.
Example 1