import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.io.BufferedWriter;
import java.util.InputMismatchException;
import java.io.IOException;
import java.io.Writer;
import java.io.OutputStreamWriter;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        OutputWriter out = new OutputWriter(outputStream);
        DMazeMaster solver = new DMazeMaster();
        solver.solve(1, in, out);
        out.close();
    }

    static class DMazeMaster {
        public void solve(int testNumber, InputReader in, OutputWriter out) {
            int h = in.readInt();
            int w = in.readInt();
            String[] s = in.readStringArray(h);
            int[][] d = new int[h * w][h * w];
            ArrayUtils.fill(d, (int) 1e8);
            for (int i = 0; i < h * w; i++) {
                d[i][i] = 0;
            }
            for (int i = 0; i < h; i++) {
                for (int j = 0; j < w; j++) {
                    if (s[i].charAt(j) == '#') continue;
                    for (int k = 0; k < 4; k++) {
                        int di = i + MiscUtils.DY4[k];
                        int dj = j + MiscUtils.DX4[k];
                        if (di < 0 || di >= h || dj < 0 || dj >= w || s[di].charAt(dj) == '#') continue;
                        d[i * w + j][di * w + dj] = 1;
                    }
                }
            }
            for (int i = 0; i < h * w; i++) {
                for (int j = 0; j < h * w; j++) {
                    for (int k = 0; k < h * w; k++) {
                        d[j][k] = Math.min(d[j][k], d[j][i] + d[i][k]);
                    }
                }
            }
            long ans = 0;
            for (int i = 0; i < h * w; i++) {
                for (int j = 0; j < h * w; j++) {
                    if (d[i][j] < (int) 1e8) ans = Math.max(ans, d[i][j]);
                }
            }
            out.printLine(ans);
        }

    }

    static class MiscUtils {
        public static final int[] DX4 = {1, 0, -1, 0};
        public static final int[] DY4 = {0, -1, 0, 1};

    }

    static class InputReader {
        private InputStream stream;
        private byte[] buf = new byte[1024];
        private int curChar;
        private int numChars;
        private InputReader.SpaceCharFilter filter;

        public InputReader(InputStream stream) {
            this.stream = stream;
        }

        public String[] readStringArray(int size) {
            String[] array = new String[size];
            for (int i = 0; i < size; i++) {
                array[i] = readString();
            }
            return array;
        }

        public int read() {
            if (numChars == -1) {
                throw new InputMismatchException();
            }
            if (curChar >= numChars) {
                curChar = 0;
                try {
                    numChars = stream.read(buf);
                } catch (IOException e) {
                    throw new InputMismatchException();
                }
                if (numChars <= 0) {
                    return -1;
                }
            }
            return buf[curChar++];
        }

        public int readInt() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            int sgn = 1;
            if (c == '-') {
                sgn = -1;
                c = read();
            }
            int res = 0;
            do {
                if (c < '0' || c > '9') {
                    throw new InputMismatchException();
                }
                res *= 10;
                res += c - '0';
                c = read();
            } while (!isSpaceChar(c));
            return res * sgn;
        }

        public String readString() {
            int c = read();
            while (isSpaceChar(c)) {
                c = read();
            }
            StringBuilder res = new StringBuilder();
            do {
                if (Character.isValidCodePoint(c)) {
                    res.appendCodePoint(c);
                }
                c = read();
            } while (!isSpaceChar(c));
            return res.toString();
        }

        public boolean isSpaceChar(int c) {
            if (filter != null) {
                return filter.isSpaceChar(c);
            }
            return isWhitespace(c);
        }

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

        public interface SpaceCharFilter {
            public boolean isSpaceChar(int ch);

        }

    }

    static class OutputWriter {
        private final PrintWriter writer;

        public OutputWriter(OutputStream outputStream) {
            writer = new PrintWriter(new BufferedWriter(new OutputStreamWriter(outputStream)));
        }

        public OutputWriter(Writer writer) {
            this.writer = new PrintWriter(writer);
        }

        public void close() {
            writer.close();
        }

        public void printLine(long i) {
            writer.println(i);
        }

    }

    static class ArrayUtils {
        public static void fill(int[][] array, int value) {
            for (int[] row : array) {
                Arrays.fill(row, value);
            }
        }

    }
}



Measured memory_bytes: 23892