#include <bits/stdc++.h>
using namespace std;
#define nl "\n"
#define nf endl
#define ll long long
#define pb push_back
#define _ << ' ' <<
#define INF (ll)1e18
#define mod 998244353
#define maxn 1010
#define maxm 11
#define maxb 1034
ll i, i1, j, k, k1, t, n, m, res, flag[10], a, b;
ll mk0, mk1, mk2, mka, dp[maxn][maxb], x;
char mt[maxn][maxm];
int main() {
ios::sync_with_stdio(0);
cin.tie(0);
#if !ONLINE_JUDGE && !EVAL
ifstream cin("input.txt");
ofstream cout("output.txt");
#endif
cin >> n >> m;
for (i = 1; i <= n; i++) {
for (j = 0; j <= m - 1; j++) {
cin >> mt[i][j];
}
}
for (i = 0; i <= n; i++) {
for (mk1 = 0; mk1 < (1 << m); mk1++) {
dp[i][mk1] = INF;
}
}
dp[0][0] = 0;
for (i = 1; i <= n; i++) {
mk0 = 0;
for (j = 0; j <= m - 1; j++) {
if (mt[i][j] == '#') mk0 += (1 << j);
}
for (mk1 = mk0;; mk1 = (mk1 - 1) & mk0) {
mka = (mk0 & (~mk1)); k = 0;
for (j = 0; j <= m - 1; j++) {
if (((mka >> j) & 1) == 0) continue;
if (j == 0 || ((mka >> (j - 1)) & 1) == 0) k++;
}
for (mk2 = mk1;; mk2 = (mk2 - 1) & mk1) {
x = __builtin_popcountll(mk1) - __builtin_popcountll(mk1 & mk2);
x += dp[i - 1][mk2];
dp[i][mk1] = min(dp[i][mk1], k + x);
if (mk2 == 0) break;
}
if (mk1 == 0) break;
}
for (mk1 = mk0;; mk1 = (mk1 - 1) & mk0) {
for (mk2 = mk1;; mk2 = (mk2 - 1) & mk1) {
dp[i][mk2] = min(dp[i][mk2], dp[i][mk1]);
if (mk2 == 0) break;
}
if (mk1 == 0) break;
}
}
/* for (i = 0; i <= n; i++) {
for (mk1 = 0; mk1 < (1 << m); mk1++) {
cout << dp[i][mk1] << ' ';
}
cout << nl;
} */
cout << dp[n][0] << nl;
return 0;
}
