from __future__ import division, print_function
_interactive = False

def main():
    n, m = input_as_list()
    g = [input() for _ in range(n)]

    from itertools import chain, groupby
    for line in chain(g, zip(*g)):
        if sum(k=="#" for k, _ in groupby(line)) > 1:
            print(-1)
            return

    if all(("#" in row) for row in g) ^ all(("#" in col) for col in zip(*g)):
        print(-1)
        return

    N = n*m
    p = list(range(N))
    r = array_of(int, N)

    def find(a):
        acopy = a
        while p[a] != a:
            a = p[a]
        while acopy != a:
            p[acopy] = p[a]
            acopy = p[acopy]
        return p[a]

    def union(a, b):
        a, b = find(a), find(b)
        if a == b: return
        if r[a] < r[b]: a, b = b, a
        if r[a] == r[b]: r[a] += 1
        p[b] = a

    for i in range(n):
        for j in range(m):
            if g[i][j] == '#':
                if i != 0 and g[i-1][j] == '#':
                    union(i*m+j, (i-1)*m+j)
                if i != n-1 and g[i+1][j] == '#':
                    union(i*m+j, (i+1)*m+j)
                if j != 0 and g[i][j-1] == '#':
                    union(i*m+j, i*m+j-1)
                if j != m-1 and g[i][j+1] == '#':
                    union(i*m+j, i*m+j+1)

    roots = set()
    for i in range(n):
        for j in range(m):
            if g[i][j] == '#':
                roots.add(find(i*m+j))

    print(len(roots))


# Constants
INF = float('inf')
MOD = 10**9+7
alphabets = 'abcdefghijklmnopqrstuvwxyz'

# Python3 equivalent names
import os, sys, itertools
if sys.version_info[0] < 3:
    input = raw_input
    range = xrange

    filter = itertools.ifilter
    map = itertools.imap
    zip = itertools.izip

# print-flush in interactive problems
if _interactive:
    flush = sys.stdout.flush()
    def printf(*args, **kwargs):
        print(*args, **kwargs)
        flush()

# Debug print, only works on local machine
LOCAL = "LOCAL_" in os.environ
debug_print = (print) if LOCAL else (lambda *x, **y: None)

# Fast IO
if (not LOCAL) and (not _interactive):
    from io import BytesIO
    from atexit import register
    sys.stdin = BytesIO(os.read(0, os.fstat(0).st_size))
    sys.stdout = BytesIO()
    register(lambda: os.write(1, sys.stdout.getvalue()))
    input = lambda: sys.stdin.readline().rstrip('\r\n')

# Some utility functions(Input, N-dimensional lists, ...)
def input_as_list():
    return [int(x) for x in input().split()]

def input_with_offset(o):
    return [int(x)+o for x in input().split()]

def input_as_matrix(n, m):
    return [input_as_list() for _ in range(n)]

def array_of(f, *dim):
    return [array_of(f, *dim[1:]) for _ in range(dim[0])] if dim else f()

main()
