Волновая трассировка

import java.util.Scanner;

public class Main {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int m = in.nextInt();
        int[][] a = new int[n + 2][m + 2];
        for (int i = 0; i < n + 2; i++) {
            a[i][0] = -1;
            a[i][m + 1] = -1;
        }
        for (int i = 0; i < m + 2; i++) {
            a[0][i] = -1;
            a[n + 1][i] = -1;
        }
        for (int i = 1; i < n + 1; i++) {
            for (int j = 1; j < m + 1; j++) {
                a[i][j] = in.nextInt();
            }
        }
        boolean f = false;
        int k = 1;
        if (a[1][1] == 0) {
            a[1][1] = k;
            while (n * m >= k) {
                for (int i = 1; i < n + 1; i++) {
                    for (int j = 1; j < m + 1; j++) {
                        if (a[i][j] == k) {
                            if (i == n && j == m) f = true;
                            if (a[i][j - 1] == 0) a[i][j - 1] = k + 1;
                            if (a[i + 1][j] == 0) a[i + 1][j] = k + 1;
                            if (a[i][j + 1] == 0) a[i][j + 1] = k + 1;
                            if (a[i - 1][j] == 0) a[i - 1][j] = k + 1;
                        }
                    }
                }
                if (f) break;
                k++;

            }
        }
        if (f) System.out.println(k - 1);
        else System.out.println(-1);
    }

}