#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<ii> vii;

#define x first
#define y second
#define pb push_back
#define eb emplace_back
#define all(v) (v).begin(), (v).end()

void run() {
	ll n, m;
	cin >> n >> m;
	vi P(n/2, 0);
	for (ll i = n-1; i >= 0; i--) {
		string line;
		cin >> line;
		for (ll j = 0; j < m; j++) {
			if (line[j] == '#') {
				P[i/2] |= 1<<j;
			}
		}
	}

	vi BC(1<<m, 0);
	for (ll i = 0; i < (1<<m); i++) {
		ll j = i;
		while (j > 0) {
			BC[i] += j%2;
			j /= 2;
		}
	}

	vvi T(n/2+1, vi(1<<m, n*m));
	ll res = n*m;
	for (ll i = 0; i < T.size(); i++) {
		for (ll j = 0; j < (1<<m); j++) {
			if (i == 0) {
				T[i][j] = 0;
				continue;
			}
			if (j%2 == 1 || (j&(j>>1)) != 0 || ((j|(j>>1))&P[i-1]) != P[i-1]) continue;
			for (ll k = 0; k < (1<<m); k++) {
				if (((k|(k<<1)|(k>>1))&j) != j) continue;
				T[i][j] = min(T[i][j], T[i-1][k] + BC[j]);
			}
			if (i == T.size() - 1 ) {
				res = min(res, T[i][j]);
			}
		}
	}
	cout << res << endl;
}

signed main() {
    ios_base::sync_with_stdio(false); cin.tie(0);
    run();
    return 0;
}