#include <bits/stdc++.h>
using namespace std;
void rset();
void init_test();
void solve();
signed main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  cout << fixed;
  cout.precision(20);
  init_test();
  return 0;
}
template <typename T>
void chmin(T& a, T b) {
  if (a > b) a = b;
}
template <typename T>
void chmax(T& a, T b) {
  if (a < b) a = b;
}
template <typename T>
void MACRO_rdv2_Init(int n, T& t) {
  t.resize(n);
}
template <typename First, typename... Rest>
void MACRO_rdv2_Init(int n, First& first, Rest&... rest) {
  first.resize(n);
  MACRO_rdv2_Init(n, rest...);
}
template <typename T>
void MACRO_rdv2_Scan(int p, T& t) {
  std::cin >> t[p];
}
template <typename First, typename... Rest>
void MACRO_rdv2_Scan(int p, First& first, Rest&... rest) {
  std::cin >> first[p];
  MACRO_rdv2_Scan(p, rest...);
}
template <typename T>
void wrv(const vector<T>& v) {
  for (int(__ii) = (0); (__ii) < (((int)v.size())); ++(__ii)) {
    if (__ii) cout << ' ';
    cout << v[__ii];
  }
  cout << '\n';
}
template <typename T>
void wrm(const vector<vector<T>>& v) {
  for (int(__ii) = (0); (__ii) < (((int)v.size())); ++(__ii)) {
    for (int(__jj) = (0); (__jj) < (v[__ii].size()); ++(__jj)) {
      if (__jj) cout << ' ';
      cout << v[__ii][__jj];
    }
    cout << '\n';
  }
}
template <typename T>
void sc(T& x) {
  cin >> x;
}
template <typename Head, typename... Tail>
void sc(Head& head, Tail&... tail) {
  cin >> head;
  sc(tail...);
}
template <typename T>
void wr(const T& x) {
  cout << x << '\n';
}
template <typename Head, typename... Tail>
void wr(const Head& head, const Tail&... tail) {
  cout << head << ' ';
  wr(tail...);
}
template <typename T>
void wrf(const T& x) {
  cout << x << endl;
}
template <typename Head, typename... Tail>
void wrf(const Head& head, const Tail&... tail) {
  cout << head << ' ';
  wrf(tail...);
}
template <typename T>
void debug_out(const T& x) {
  cerr << x << '\n';
}
template <typename Head, typename... Tail>
void debug_out(const Head& head, const Tail&... tail) {
  cerr << head << ' ';
  debug_out(tail...);
}
template <typename... T>
void err(const T&... cod) {
  wr(cod...);
  exit(0);
}
const int N = 1005;
const int dy[] = {-1, 0, 1, 0};
const int dx[] = {0, 1, 0, -1};
int n, m;
string a[N];
bool banned[N][N];
bool vis[N][N];
bool valid(int y, int x) { return (min(y, x) >= 0 && y < n && x < m); }
void dfs(int y, int x) {
  vis[y][x] = true;
  for (int(d) = (0); (d) < (4); ++(d)) {
    int ny = y + dy[d];
    int nx = x + dx[d];
    if (valid(ny, nx) && !vis[ny][nx] && a[ny][nx] == '#') dfs(ny, nx);
  }
}
void solve() {
  cin >> n >> m;
  for (int(i) = (0); (i) < (n); ++(i)) cin >> a[i];
  for (int(i) = (0); (i) < (n); ++(i)) {
    int j = 0;
    while (j < m) {
      if (a[i][j] == '.') {
        ++j;
      } else {
        int fi = j;
        while (j < m && a[i][j] == a[i][fi]) ++j;
        --j;
        for (int(k) = (0); (k) < (m); ++(k)) {
          if (k < fi || k > j) banned[i][k] = true;
        }
        break;
      }
    }
  }
  for (int(j) = (0); (j) < (m); ++(j)) {
    int i = 0;
    while (i < n) {
      if (a[i][j] == '.') {
        ++i;
      } else {
        int fi = i;
        while (i < n && a[i][j] == a[fi][j]) ++i;
        --i;
        for (int(k) = (0); (k) < (n); ++(k)) {
          if (k < fi || k > i) banned[k][j] = true;
        }
        break;
      }
    }
  }
  for (int(i) = (0); (i) < (n); ++(i))
    for (int(j) = (0); (j) < (m); ++(j)) {
      if (banned[i][j] && a[i][j] == '#') {
        err(-1);
      }
    }
  set<int> sr, sc;
  for (int(i) = (0); (i) < (n); ++(i))
    for (int(j) = (0); (j) < (m); ++(j)) {
      if (!banned[i][j]) {
        sr.insert(i);
        sc.insert(j);
      }
    }
  if (((int)sr.size()) < n || ((int)sc.size()) < m) err(-1);
  int ans = 0;
  for (int(i) = (0); (i) < (n); ++(i))
    for (int(j) = (0); (j) < (m); ++(j)) {
      if (!vis[i][j] && a[i][j] == '#') {
        ++ans;
        dfs(i, j);
      }
    }
  wr(ans);
}
void init_test() {
  int qq = 1;
  while (qq--) solve();
}
