import java.util.*;
import java.io.*;
import java.text.*;

public class Main{
    //SOLUTION BEGIN
    //Into the Hardware Mode
    void pre() throws Exception{}
    void solve(int TC) throws Exception {
        int n = ni(), m = ni();
        String[] grid = new String[n];
        int[] minR = new int[n], maxR = new int[n];
        int[] minC = new int[m], maxC = new int[m];
        for(int i = 0; i< n; i++)Arrays.fill(minR, m+1);
        for(int i = 0; i< m; i++)Arrays.fill(minC, n+1);
        int[] cntR = new int[n], cntC = new int[m];
        for(int i = 0; i< n; i++){
            grid[i] = n();
            for(int j = 0; j< m; j++){
                if(grid[i].charAt(j) == '#'){
                    minR[i] = Math.min(minR[i], j);
                    maxR[i] = Math.max(maxR[i], j);
                    minC[j] = Math.min(minC[j], i);
                    maxC[j] = Math.max(maxC[j], i);
                    cntR[i]++;cntC[j]++;
                }
            }
        }
        boolean valid = true;
        boolean er = false, ec = false;
        for(int i = 0; i< n; i++)
            if(cntR[i] != 0)
                valid &= (maxR[i]-minR[i]+1 == cntR[i]);
            else er = true;
        for(int i = 0; i< m; i++)
            if(cntC[i] != 0)
                valid &= (maxC[i]-minC[i]+1 == cntC[i]);
            else ec = true;
        if(er^ec)valid = false;
        if(!valid){
            pn(-1);
            return;
        }
        int[][] D = new int[][]{
                {-1, 0},
                {1, 0},
                {0, -1},
                {0, 1}
        };
        int[] set = new int[n*m];
        for(int i = 0; i< n*m; i++)set[i] = i;
        for(int i = 0; i< n; i++){
            for(int j = 0; j< m; j++){
                if(grid[i].charAt(j) != '#')continue;
                for(int[] d:D){
                    int ii = i+d[0], jj = j+d[1];
                    if(ii < 0 || jj < 0 || ii >= n || jj >= m || grid[ii].charAt(jj) != grid[i].charAt(j))continue;
                    set[find(set, ii*m+jj)] = find(set, i*m+j);
                }
            }
        }
        int ans = 0;
        for(int i = 0; i< n; i++)
            for(int j = 0; j< m; j++)
                if(grid[i].charAt(j) == '#' && find(set, i*m+j) == i*m+j)
                    ans++;

        pn(ans);
    }
    //SOLUTION END
    static void debug(Object...obj){System.err.println(Arrays.deepToString(obj)); }
    void hold(boolean b)throws Exception{if(!b)throw new Exception("Hold right there, Sparky!");}
    void exit(boolean b){if(!b)System.exit(0);}
    final long IINF = (long)2e18;
    final int INF = (int)1e9+2, MX = (int)2e6+5;
    DecimalFormat df = new DecimalFormat("0.00000000000");
    double PI = 3.141592653589793238462643383279502884197169399, eps = 1e-8;
    static boolean multipleTC = false, memory = false, fileIO = false;
    FastReader in;PrintWriter out;
    void run() throws Exception{
        long ct = System.currentTimeMillis();
        if (fileIO) {
            in = new FastReader("");
            out = new PrintWriter("");
        } else {
            in = new FastReader();
            out = new PrintWriter(System.out);
        }
        //Solution Credits: Taranpreet Singh
        int T = multipleTC? ni():1;
        pre();
        for (int t = 1; t <= T; t++) solve(t);
        out.flush();
        out.close();
        System.err.println(System.currentTimeMillis() - ct);
    }
    public static void main(String[] args) throws Exception{
        if(memory)new Thread(null, new Runnable() {public void run(){try{new Main().run();}catch(Exception e){e.printStackTrace();}}}, "1", 1 << 28).start();
        else new Main().run();
    }
    int[][] make(int n, int[] from, int[] to, int e, boolean f){
        int[][] g = new int[n][];int[]cnt = new int[n];
        for(int i = 0; i< e; i++){
            cnt[from[i]]++;
            if(f)cnt[to[i]]++;
        }
        for(int i = 0; i< n; i++)g[i] = new int[cnt[i]];
        for(int i = 0; i< e; i++){
            g[from[i]][--cnt[from[i]]] = to[i];
            if(f)g[to[i]][--cnt[to[i]]] = from[i];
        }
        return g;
    }
    int[][][] makeS(int n, int[] from, int[] to, boolean f){
        int[][][] g = new int[n][][];int[]cnt = new int[n];
        for(int i:from)cnt[i]++;if(f)for(int i:to)cnt[i]++;
        for(int i = 0; i< n; i++)g[i] = new int[cnt[i]][];
        for(int i = 0; i< from.length; i++){
            g[from[i]][--cnt[from[i]]] = new int[]{to[i], i};
            if(f)g[to[i]][--cnt[to[i]]] = new int[]{from[i], i};
        }
        return g;
    }
    int find(int[] set, int u){return set[u] = (set[u] == u?u:find(set, set[u]));}
    int digit(long s){int ans = 0;while(s>0){s/=10;ans++;}return ans;}
    long gcd(long a, long b){return (b==0)?a:gcd(b,a%b);}
    int gcd(int a, int b){return (b==0)?a:gcd(b,a%b);}
    int bit(long n){return (n==0)?0:(1+bit(n&(n-1)));}
    void p(Object o){out.print(o);}
    void pn(Object o){out.println(o);}
    void pni(Object o){out.println(o);out.flush();}
    String n()throws Exception{return in.next();}
    String nln()throws Exception{return in.nextLine();}
    int ni()throws Exception{return Integer.parseInt(in.next());}
    long nl()throws Exception{return Long.parseLong(in.next());}
    double nd()throws Exception{return Double.parseDouble(in.next());}

    class FastReader{
        BufferedReader br;
        StringTokenizer st;
        public FastReader(){
            br = new BufferedReader(new InputStreamReader(System.in));
        }

        public FastReader(String s) throws Exception{
            br = new BufferedReader(new FileReader(s));
        }

        String next() throws Exception{
            while (st == null || !st.hasMoreElements()){
                try{
                    st = new StringTokenizer(br.readLine());
                }catch (IOException  e){
                    throw new Exception(e.toString());
                }
            }
            return st.nextToken();
        }

        String nextLine() throws Exception{
            String str;
            try{
                str = br.readLine();
            }catch (IOException e){
                throw new Exception(e.toString());
            }
            return str;
        }
    }
}