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#include <bits/stdc++.h>
using namespace std;
#define ll long long
#define endl "\n"
const int N = 10;
const int p = 1e9 + 7;
ll dp[1001][11][1<<10];
vector<string> vec(1001);
void solve(){
int n, m;
cin >> n >> m;
int mask = 1 << m;
for(int i = 0; i < m; i++){
vec[0] += '.';
}
for(int i = 1; i <= n; i++){
cin >> vec[i];
}
for(int i = 0; i < mask; i++){
dp[0][m][i] = 0;
}
for(int i = 1; i <= n; i++){
for(int j = 1; j <= m; j++){
for(int k = 0; k < mask; k++){
int nm = k ^ (1 << (j - 1));
if(vec[i][j - 1] == '#'){
if(k & (1 << (j - 1))){
if(j == 1){
dp[i][j][k] = min(dp[i - 1][m][k], dp[i - 1][m][nm]) + 1;
if(vec[i - 1][j - 1] == '#'){
dp[i][j][k] = min(dp[i][j][k], dp[i - 1][m][k]);
}
}else{
dp[i][j][k] = min(dp[i][j - 1][k], dp[i][j - 1][nm]) + 1;
if(vec[i - 1][j - 1] == '#'){
dp[i][j][k] = min(dp[i][j][k], dp[i][j - 1][k]);
}
}
}else{
if(j == 1){
dp[i][j][k] = min(dp[i - 1][m][k], dp[i - 1][m][nm]) + 1;
}else{
dp[i][j][k] = min(dp[i][j - 1][k], dp[i][j - 1][nm]) + 1;
if(vec[i][j - 2] == '#' && (k & (1 << (j - 2))) == 0){
dp[i][j][k] -= 1;
}
}
}
}else{
if(j == 1){
dp[i][j][k] = min(dp[i - 1][m][k], dp[i - 1][m][nm]);
}else{
dp[i][j][k] = min(dp[i][j - 1][k], dp[i][j - 1][nm]);
}
}
}
}
}
ll res = n*m;
for(int i = 0; i < mask; i++){
res = min(res, dp[n][m][i]);
}
cout << res << endl;
}
int main()
{
ios::sync_with_stdio(false);
cin.tie(NULL);
int t = 1;
while(t--){
solve();
}
return 0;
}
