import java.io.IOException;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.LinkedList;
import java.util.List;

public class Main {
	List<Integer>[] e;
	boolean[] visit;

	boolean dfs(int pre, int k) {
		if (visit[k]) {
			return true;
		}
		visit[k] = true;
		for (Integer to : e[k]) {
			if (to == pre) {
				continue;
			}
			if (dfs(k, to)) {
				return true;
			}
		}
		return false;
	}

	void run() {
		MyScanner sc = new MyScanner();

		int w = sc.nextInt();
		int h = sc.nextInt();
		int[][] m = new int[h][w];
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				m[i][j] = sc.nextInt();
			}
		}

		int[] dx = { -1, 0, 0, 1 };
		int[] dy = { 0, -1, 1, 0 };
		e = new LinkedList[w * h];
		for (int i = 0; i < w * h; i++) {
			e[i] = new LinkedList<Integer>();
		}
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				for (int k = 0; k < 4; k++) {
					int nx = j + dx[k];
					int ny = i + dy[k];
					if (nx < 0 || w <= nx || ny < 0 || h <= ny) {
						continue;
					}
					if (m[i][j] == m[ny][nx]) {
						int s = i * w + j;
						int t = ny * w + nx;
						e[s].add(t);
					}
				}
			}
		}

		boolean f = false;
		visit = new boolean[w * h];
		for (int i = 0; i < h; i++) {
			for (int j = 0; j < w; j++) {
				Arrays.fill(visit, false);
				if (dfs(-1, i * w + j)) {
					f = true;
					break;
				}
			}
		}

		System.out.println(f ? "possible" : "impossible");
	}

	public static void main(String[] args) {
		new Main().run();
	}

	public void mapDebug(int[][] a) {
		System.out.println("--------map display---------");
		for (int i = 0; i < a.length; i++) {
			for (int j = 0; j < a[i].length; j++) {
				System.out.printf("%3d ", a[i][j]);
			}
			System.out.println();
		}
		System.out.println("----------------------------" + '\n');
	}

	class MyScanner {
		int read() {
			try {
				return System.in.read();
			} catch (IOException e) {
				throw new InputMismatchException();
			}
		}

		boolean isSpaceChar(int c) {
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}

		boolean isEndline(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}

		int nextInt() {
			return Integer.parseInt(next());
		}

		int[] nextIntArray(int n) {
			int[] array = new int[n];
			for (int i = 0; i < n; i++)
				array[i] = nextInt();
			return array;
		}

		long nextLong() {
			return Long.parseLong(next());
		}

		long[] nextLongArray(int n) {
			long[] array = new long[n];
			for (int i = 0; i < n; i++)
				array[i] = nextLong();
			return array;
		}

		double nextDouble() {
			return Double.parseDouble(next());
		}

		double[] nextDoubleArray(int n) {
			double[] array = new double[n];
			for (int i = 0; i < n; i++)
				array[i] = nextDouble();
			return array;
		}

		String next() {
			int c = read();
			while (isSpaceChar(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isSpaceChar(c));
			return res.toString();
		}

		String[] nextStringArray(int n) {
			String[] array = new String[n];
			for (int i = 0; i < n; i++)
				array[i] = next();

			return array;
		}

		String nextLine() {
			int c = read();
			while (isEndline(c))
				c = read();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(c);
				c = read();
			} while (!isEndline(c));
			return res.toString();
		}
	}
}