Manhattan distance is like walking through a city grid: you can move horizontally and vertically, but not diagonally. Given two points (x1, y1) and (x2, y2), return the distance between them using abs(x1 - x2) + abs(y1 - y2). This is useful for maps, grid-based games, and route calculations on rectangular layouts.

def manhattan(x1, y1, x2, y2):