## Problem Description

Given a graph composed of n * m grids , the color of each grid is either black or white. Every step of gragh transform operation can transform a subgraph B which is size of r * c within graph A,and let all the grids in graph B turn into its opposite color. Please design of an algorithm to calculate the minimal number of steps can transform graph A into an all white graph.

## Input

There are multiple test cases. The first line of each test case contains four positive integers n,m,r,c. (1 ≤ r ≤ n ≤ 100, 1 ≤ c ≤ m ≤ 100). indicating that graph A has n rows ,m columns and subgraph B has r rows,c columns. In the following n lines, each line contains a m-length 01string, indicating the graph A,0 for white and 1 for black.

## Output

For each test case, print a line containing an integer which indicting the minimal transform steps, if it is impossible to transform,please output " No solution!".

## Sample Input

3 4 1 1
0110
0111
0000
4 8 1 7
10000001
10000001
10000001
11111110
2 8 1 4
10010111
01010101

## Sample Output

5
7
No solution!

## Source

FZU 2007 ICPC Qualification Round I