#include <bits/stdc++.h>
using namespace std;
const int inf = 1e9 + 7;
bool arr[1001][11];
int dp[1001][11][1024];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(0);
int rows, cols, p2c; cin >> rows >> cols;
p2c = 1 << cols;
for (int r = 1; r <= rows; r++){
for (int c = 0; c < cols; c++){
char ch; cin >> ch;
arr[r][c] = (ch == '#');
}
}
dp[0][cols - 1][0] = 0;
for (int b = 1; b < p2c; b++) dp[0][cols - 1][b] = inf;
int cm = 0;
for (int r = 1; r <= rows; r++){
for (int c = 0; c < cols; c++){
if (arr[r][c] != arr[r - 1][c]) cm ^= (1 << c);
for (int b = 0; b < p2c; b++){
if ((cm | b) != cm) dp[r][c][b] = inf;
else if (c == 0){
if (b & (1 << c)) dp[r][c][b] = min(dp[r - 1][cols - 1][b], dp[r - 1][cols - 1][b ^ (1 << c)] + 1);
else dp[r][c][b] = min(dp[r - 1][cols - 1][b], dp[r - 1][cols - 1][b ^ (1 << c)]) + arr[r][c];
}
else{
if (b & (1 << c)) dp[r][c][b] = min(dp[r][c - 1][b], dp[r][c - 1][b ^ (1 << c)] + 1);
else dp[r][c][b] = min(dp[r][c - 1][b], dp[r][c - 1][b ^ (1 << c)]) + (arr[r][c] && (!arr[r][c - 1] || (b & (1 << (c - 1)))));
}
}
}
}
int ans = inf;
for (int b = 0; b < p2c; b++) ans = min(ans, dp[rows][cols - 1][b]);
cout << ans;
}
