#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define MASK(i) (1LL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
#define ALL(v) (v).begin(), (v).end()
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll LASTBIT(ll mask){return (mask) & (-mask);}
int pop_cnt(ll mask){return __builtin_popcountll(mask);}
int ctz(ll mask){return __builtin_ctzll(mask);}
int logOf(ll mask){return 63 - __builtin_clzll(mask);}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
ll rngesus(ll l, ll r){return l + (ull) rng() % (r - l + 1);}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T& container, char separator = ' ', char finish = '\n'){
for(auto item: container) cout << item << separator;
cout << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
const int N = 1010;
const int INF = 1e9 + 69;
int n, m;
string s[N];
int dp[N][MASK(10)];
int continuous_cnt[MASK(10)];
int diff[MASK(10)][MASK(10)];
struct P{
int i, w;
P(){}
P(int i, int w): i(i), w(w){}
};
struct compare{
bool operator () (P a, P b){
return a.w > b.w;
}
};
int main(void){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n >> m;
for(int i = 0; i<n; ++i) cin >> s[i];
for(int mask = 0; mask < MASK(m); ++mask){
continuous_cnt[mask] = pop_cnt(mask);
for(int i = 1; i < m; ++i) if (GETBIT(mask, i) && GETBIT(mask, i-1))
continuous_cnt[mask]--;
}
for(int mask1 = 0; mask1 < MASK(m); ++mask1)
for(int mask2 = 0; mask2 < MASK(m); ++mask2){
for(int i = 0; i < m; ++i) if (GETBIT(mask2, i) && !GETBIT(mask1, i))
diff[mask1][mask2]++;
}
for(int i = 0; i<=n; ++i) for(int j = 0; j < MASK(m); ++j) dp[i][j] = INF;
dp[0][0] = 0;
for(int i = 0; i<n; ++i){
int cur_layer = 0;
for(int j = 0; j < m; ++j) if (s[i][j] == '#') cur_layer += MASK(j);
priority_queue<P, vector<P>, compare> pq;
for(int j = 0; j < MASK(m); ++j) if (dp[i][j] != INF) pq.push(P(j, dp[i][j]));
while(pq.size()){
P u = pq.top(); pq.pop();
for(int j = 0; j < m; ++j) {
int cost = u.w + (GETBIT(u.i, j) == 0);
if (minimize(dp[i][u.i ^ MASK(j)], cost))
pq.push(P(u.i ^ MASK(j), cost));
}
}
for(int j = cur_layer; j >= 0; j = (j - 1) & cur_layer) if (dp[i][j] != INF){
int cost = continuous_cnt[cur_layer - j] + dp[i][j];
minimize(dp[i+1][j], cost);
if (j == 0) break;
}
}
int ans = INF;
for(int mask = 0; mask < MASK(m); ++mask) minimize(ans, dp[n][mask]);
cout << ans << "\n";
return 0;
}
