// package atcoder.other2016.kupc2016;

import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.InputMismatchException;
import java.util.List;

public class Main {
    private static final int INF = 1000000;

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

    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        PrintWriter out = new PrintWriter(System.out);

        int h = in.nextInt();
        int w = in.nextInt();
        char[][] map = new char[h][];
        for (int i = 0; i < h ; i++) {
            map[i] = in.nextToken().toCharArray();
        }
        boolean has = false;
        for (int i = 0; i < h; i++) {
            has |= map[i][0] == 'X' || map[i][w-1] == 'X';
        }
        for (int i = 0; i < w ; i++) {
            has |= map[0][i] == 'X' || map[h-1][i] == 'X';
        }
        if (has) {
            out.println(-1);
        } else {
            out.println(solve(map));
        }
        out.flush();
    }

    private static int solve(char[][] map) {
        int h = map.length;
        int w = map[0].length;
        int source = 2*h*w;
        int sink = source+1;
        MaxFlowDinic flow = new MaxFlowDinic();
        flow.init(sink+1);

        for (int i = 0 ; i < h ; i++) {
            for (int j = 0; j < w ; j++) {
                int vin = (i*w+j)*2;
                int vout = vin+1;

                if (map[i][j] == 'X') {
                    flow.edge(source, vin, INF);
                    flow.edge(vin, vout, INF);
                } else {
                    flow.edge(vin, vout, 1);
                }
                for (int d = 0 ; d < 4 ; d++) {
                    int ti = i + DY[d];
                    int tj = j + DX[d];
                    if (ti < 0 || tj < 0 || ti >= h || tj >= w) {
                        flow.edge(vout, sink, INF);
                    } else {
                        int tvin = (ti*w+tj)*2;
                        flow.edge(vout, tvin, INF);
                    }
                }
            }
        }
        return flow.max_flow(source, sink);
    }

    public static class MaxFlowDinic {
        public List<int[]>[] graph;
        public int[] deg;

        public int[] level;
        public int[] itr;

        public int[] que;

        @SuppressWarnings("unchecked")
        public void init(int size) {
            graph = new List[size];
            for (int i = 0; i < size ; i++) {
                graph[i] = new ArrayList<int[]>();
            }
            deg = new int[size];
            level = new int[size];
            itr = new int[size];
            que = new int[size+10];
        }
        public void edge(int from, int to, int cap) {
            int fdeg = deg[from];
            int tdeg = deg[to];
            graph[from].add(new int[]{to, cap, tdeg});
            graph[to].add(new int[]{from, 0, fdeg});
            deg[from]++;
            deg[to]++;
        }

        public int dfs(int v, int t, int f) {
            if (v == t) return f;
            for (int i = itr[v] ; i < graph[v].size() ; i++) {
                itr[v] = i;
                int[] e = graph[v].get(i);
                if (e[1] > 0 && level[v] < level[e[0]]) {
                    int d = dfs(e[0], t, Math.min(f, e[1]));
                    if (d > 0) {
                        e[1] -= d;
                        graph[e[0]].get(e[2])[1] += d;
                        return d;
                    }
                }
            }
            return 0;
        }

        public void bfs(int s) {
            Arrays.fill(level, -1);
            int qh = 0;
            int qt = 0;
            level[s] = 0;
            que[qh++] = s;
            while (qt < qh) {
                int v = que[qt++];
                for (int i = 0; i < graph[v].size() ; i++) {
                    int[] e = graph[v].get(i);
                    if (e[1] > 0 && level[e[0]] < 0) {
                        level[e[0]] = level[v] + 1;
                        que[qh++] = e[0];
                    }
                }
            }
        }

        public int max_flow(int s, int t) {
            int flow = 0;
            while (true) {
                bfs(s);
                if (level[t] < 0) {
                    return flow;
                }
                Arrays.fill(itr, 0);
                while (true) {
                    int f = dfs(s, t, Integer.MAX_VALUE);
                    if (f <= 0) {
                        break;
                    }
                    flow += f;
                }
            }
        }
    }

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

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

        private int[] nextInts(int n) {
            int[] ret = new int[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextInt();
            }
            return ret;
        }


        private int[][] nextIntTable(int n, int m) {
            int[][] ret = new int[n][m];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < m; j++) {
                    ret[i][j] = nextInt();
                }
            }
            return ret;
        }

        private long[] nextLongs(int n) {
            long[] ret = new long[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextLong();
            }
            return ret;
        }

        private long[][] nextLongTable(int n, int m) {
            long[][] ret = new long[n][m];
            for (int i = 0; i < n; i++) {
                for (int j = 0; j < m; j++) {
                    ret[i][j] = nextLong();
                }
            }
            return ret;
        }

        private double[] nextDoubles(int n) {
            double[] ret = new double[n];
            for (int i = 0; i < n; i++) {
                ret[i] = nextDouble();
            }
            return ret;
        }

        private int next() {
            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 char nextChar() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            if ('a' <= c && c <= 'z') {
                return (char) c;
            }
            if ('A' <= c && c <= 'Z') {
                return (char) c;
            }
            throw new InputMismatchException();
        }

        public String nextToken() {
            int c = next();
            while (isSpaceChar(c))
                c = next();
            StringBuilder res = new StringBuilder();
            do {
                res.append((char) c);
                c = next();
            } while (!isSpaceChar(c));
            return res.toString();
        }

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

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

        public double nextDouble() {
            return Double.valueOf(nextToken());
        }

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

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

    static void debug(Object... o) {
        System.err.println(Arrays.deepToString(o));
    }
}


Measured memory_bytes: 15112