Shortest Path in City Blocks

You are given a grid representing streets in a city where each cell contains either '0' (open street) or '1' (building). You start from the top-left corner (0,0) and want to reach the bottom-right corner (n-1,m-1) by moving only along open streets. Find the shortest path in terms of the number of moves required. If no path exists, return -1. Input: The first line contains two integers n and m (1 <= n, m <= 1000), the dimensions of the grid. The following n lines each contain m characters ('0' or '1'). Output: An integer representing the shortest path length or -1 if no path exists. Example: Input: 3 3 0 1 0 0 0 0 1 0 0 Output: 4