#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using ld = long double;
using pl = pair<ll,ll>;
using pii = pair<int,int>;
using tpl = tuple<int,int,int>;
#define all(a) a.begin(), a.end()
#define filter(a) a.erase(unique(all(a)), a.end())
int dp[2][1 << 11][11];
char ch[1010][11];
bool getCur (int mask, int pos) {
return mask >> pos & 1;
}
int main()
{
ios::sync_with_stdio(0);
cin.tie(0);
/// read input
int n, m; cin >> n >> m;
for (int i = 1; i <= n; i++) {
for (int j = 1; j <= m; j++) cin >> ch[i][j];
ch[i][0] = ch[i][m + 1] = '.';
}
for (int j = 0; j <= m + 1; j++) ch[n + 1][j] = '.';
/// base cases
for (int mask = 1; mask < (1 << m); mask++)
dp[0][mask << 1][m] = INT_MAX;
/// broken profile DP
for (int i = 1; i <= n + 1; i++) {
int t = i & 1;
// refill state
for (int msk = 0; msk < (1 << m); msk++)
fill(dp[t][msk << 1], dp[t][msk << 1] + m + 1, INT_MAX);
// compute first state
for (int msk = 0; msk < (1 << m); msk++)
dp[t][msk << 1][0] = dp[t ^ 1][msk << 1][m];
// compute other state
for (int j = 1; j <= m; j++) {
for (int msk = 0; msk < (1 << m); msk++) {
int mask = msk << 1;
if (getCur(mask, j) && ch[i][j] == '.') continue;
if (getCur(mask, j)) // column j covered
dp[t][mask][j] = min(dp[t][mask][j - 1], dp[t][mask ^ (1 << j)][j - 1]);
else {
char pre = (getCur(mask, j - 1) ? '.' : ch[i][j - 1]);
int cost = (ch[i][j] == '#' && pre == '.');
if (dp[t][mask][j - 1] != INT_MAX)
dp[t][mask][j] = min(dp[t][mask][j], dp[t][mask][j - 1] + cost);
if (dp[t][mask ^ (1 << j)][j - 1] != INT_MAX)
dp[t][mask][j] = min(dp[t][mask][j], dp[t][mask ^ (1 << j)][j - 1] + cost + 1);
}
}
}
// cout << "table " << i << ":\n";
// for (int j = 0; j <= m; j++) {
// for (int mask = 0; mask < (1 << m); mask++) cout << dp[t][mask << 1][j] << " ";
// cout << "\n";
// }
}
cout << dp[(n + 1) & 1][0][m];
return 0;
}
