#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9;
int dp[1000][10][1 << 10][2];
bool a[1000][10];
int n, m;
int checkbit (int mask, int x) {
return (mask >> x) & 1;
}
int off (int mask, int x) {
if (checkbit(mask, x)) mask ^= 1 << x;
return mask;
}
int ans (int pos, int cur, int mask, int c) {
if (pos == n) return 0;
if (cur == m) {
return ans(pos + 1, 0, mask, 0);
}
if (a[pos][cur] == 0) {
return ans(pos, cur + 1, off(mask, cur), 0);
}
int &ret = dp[pos][cur][mask][c];
if (ret != -1) return ret;
ret = inf;
if (checkbit(mask, cur)) {
ret = min(ret, ans(pos, cur + 1, mask, 0));
} else {
ret = min(ret, 1 + ans(pos, cur + 1, mask ^ (1 << cur), 0));
}
if (c) {
ret = min(ret, ans(pos, cur + 1, off(mask, cur), 1));
} else {
ret = min(ret, 1 + ans(pos, cur + 1, off(mask, cur), 1));
}
return ret;
}
int main () {
memset(dp, -1, sizeof(dp));
cin >> n >> m;
for (int i = 0; i < n; i++) {
for (int j = 0; j < m; j++) {
char c;
cin >> c;
a[i][j] = c == '#';
}
}
cout << ans(0, 0, 0, 0) << '\n';
}
