240905 연결

static class Point {
	int x;
	int y;

	public Point(int x, int y) {
		this.x = x;
		this.y = y;
	}
}

static final String IMPOSSIBLE = "IMPOSSIBLE";
static Point[][] prev;
static int[][] visited;
static int n;
static int m;

static final int[] DX = { -1, 1, 0, 0 };
static final int[] DY = { 0, 0, -1, 1 };

public static void main(String[] args) throws IOException {
	BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	StringTokenizer st = new StringTokenizer(br.readLine());
	m = Integer.parseInt(st.nextToken());
	n = Integer.parseInt(st.nextToken());

	Point[] a = new Point[2];
	Point[] b = new Point[2];

	for (int i = 0; i < 2; i++) {
		st = new StringTokenizer(br.readLine());
		int x = Integer.parseInt(st.nextToken());
		int y = Integer.parseInt(st.nextToken());
		a[i] = new Point(y, x);
	}

	for (int i = 0; i < 2; i++) {
		st = new StringTokenizer(br.readLine());
		int x = Integer.parseInt(st.nextToken());
		int y = Integer.parseInt(st.nextToken());
		b[i] = new Point(y, x);
	}

	int answer = calculateMinimumDistance(a, b);
	answer = Math.min(answer, calculateMinimumDistance(b, a));

	if (answer == Integer.MAX_VALUE) {
		System.out.println(IMPOSSIBLE);
	} else {
		System.out.println(answer);
	}
}

private static int calculateMinimumDistance(Point[] p1, Point[] p2) {
	visited = new int[n + 1][m + 1];
	visited[p2[0].x][p2[0].y] = visited[p2[1].x][p2[1].y] = 1;
	prev = new Point[n + 1][m + 1];

	bfs(p1);
	int aDistance = visited[p1[1].x][p1[1].y];

	if (aDistance == 0)
		return Integer.MAX_VALUE;

	visited = new int[n + 1][m + 1];
	Point current = prev[p1[1].x][p1[1].y];

	while (current != null && (current.x != p1[0].x || current.y != p1[0].y)) {
		visited[current.x][current.y] = 1;
		current = prev[current.x][current.y];
	}

	visited[p1[0].x][p1[0].y] = visited[p1[1].x][p1[1].y] = 1;
	bfs(p2);
	int bDistance = visited[p2[1].x][p2[1].y];

	return bDistance == 0 ? Integer.MAX_VALUE : aDistance + bDistance;
}

private static void bfs(Point[] points) {
	Queue<Point> queue = new LinkedList<>();
	queue.add(points[0]);

	while (!queue.isEmpty()) {
		Point current = queue.poll();

		for (int i = 0; i < 4; i++) {
			int newX = current.x + DX[i];
			int newY = current.y + DY[i];

			if (!isInBounds(newX, newY) || visited[newX][newY] != 0) {
				continue;
			}

			queue.add(new Point(newX, newY));
			visited[newX][newY] = visited[current.x][current.y] + 1;
			prev[newX][newY] = current;

			if (newX == points[1].x && newY == points[1].y) {
				return;
			}
		}
	}
}

private static boolean isInBounds(int x, int y) {
	return x >= 0 && x <= n && y >= 0 && y <= m;
}

출처:https://www.acmicpc.net/problem/5022