import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

from collections import deque

"""
・最小カット
・頂点でカットする場合には、v_in, v_outに分ける
・v_in, v_outの間に容量1の辺
・sourceから、ヤギ_outに容量無限大の辺
・外周_outから、sinkに容量無限大の辺
"""

H,W = map(int,readline().split())
S = b''.join(read().split())

class Dinic:
    def __init__(self, N, source, sink):
        self.N = N
        self.G = [[] for _ in range(N)]
        self.source = source
        self.sink = sink

    def add_edge(self, fr, to, cap):
        n1 = len(self.G[fr])
        n2 = len(self.G[to])
        self.G[fr].append([to, cap, n2])
        self.G[to].append([fr, 0, n1]) # 逆辺を cap 0 で追加
        
    def add_edge_undirected(self, fr, to, cap):
        n1 = len(self.G[fr])
        n2 = len(self.G[to])
        self.G[fr].append([to, cap, n2])
        self.G[to].append([fr, cap, n1])
        
    def bfs(self):
        level = [0] * self.N
        G = self.G; source = self.source; sink = self.sink
        q = deque([source])
        level[source] = 1
        pop = q.popleft; append = q.append
        while q:
            v = pop()
            lv = level[v] + 1
            for to, cap, rev in G[v]:
                if not cap:
                    continue
                if level[to]:
                    continue
                level[to] = lv
                if to == sink:
                    self.level = level
                    return
                append(to)
        self.level = level
        
    def dfs(self):
        INF = 10**18
        G = self.G
        sink = self.sink
        prog = self.progress
        level = self.level
        ascend = False
        ff = 0
        stack = [(self.source,INF)]
        while stack:
            if ff:
                # このまま更新だけして戻っていく
                v,f = stack.pop()
                p = prog[v]
                to, cap, rev = G[v][p]
                G[v][p][1] -= ff
                G[to][rev][1] += ff
                continue
            v,f = stack[-1]
            if v == sink:
                ff = f
                stack.pop()
                continue
            E = G[v]
            lv = level[v]
            if ascend:
                # 流せずに戻ってきた
                prog[v] += 1
            find_flag = False
            for i in range(prog[v],len(E)):
                to, cap, rev = E[i]
                prog[v] = i
                if not cap:
                    continue
                if level[to] <= lv:
                    continue
                find_flag = True
                break
            if not find_flag:
                ascend = True
                stack.pop()
                continue
            ascend = False
            x = f if f < cap else cap
            stack.append((to,x))
        return ff
    
    def max_flow(self):
        flow = 0
        while True:
            self.bfs()
            if not self.level[self.sink]:
                return flow
            self.progress = [0] * self.N
            while True:
                f = self.dfs()
                if not f:
                    break
                flow += f
        return flow

# v_i-in: 2i
# v_i-out: 2i+1
source = 2*H*W
sink = source+1

INF = 10**12

dinic = Dinic(2*H*W+2,source,sink)
add = dinic.add_edge
for i in range(H*W):
    h,w = divmod(i,W)
    if S[i] == ord('.'):
        add(i+i,i+i+1,1)
    else:
        add(source,i+i+1,INF)
    for dh,dw in ((1,0),(-1,0),(0,1),(0,-1)):
        h1 = h + dh; w1 = w + dw
        if 0 <= h1 < H and 0 <= w1 < W:
            j = h1 * W + w1
            add(i+i+1,j+j,1)
        else:
            # 画面外
            add(i+i+1,sink,INF)

dinic.source

f = dinic.max_flow()
answer = f if f < INF else -1
print(answer)



Measured memory_bytes: 19288