#include <bits/stdc++.h>
using namespace std;
template<typename T>
bool minimize(T& a, const T& b){
if(a > b) return a = b, true;
return false;
}
int main(){
ios_base::sync_with_stdio(0); cin.tie(0);
#ifdef LOCAL
freopen("task.inp", "r", stdin);
freopen("task.out", "w", stdout);
#endif // LOCAL
int N, M;
cin >> N >> M;
vector<vector<char>> a(N, vector<char>(M));
for(int i = 0; i < N; ++i){
for(int j = 0; j < M; ++j){
cin >> a[i][j];
}
}
const int inf = 1e9;
vector<vector<vector<int>>> f(N + 1, vector<vector<int>>(M + 1, vector<int>(1 << M, inf)));
f[0][0][0] = 0;
//mask : use to horizontal tapes some rows
for(int i = 0; i < N; ++i){
for(int j = 0; j < M; ++j){
for(int mask = 0; mask < (1 << M); ++mask){
if(f[i][j][mask] == inf) continue;
if(a[i][j] == '#'){
if(mask >> j & 1){
minimize(f[i][j + 1][mask], f[i][j][mask]);
} else{
minimize(f[i][j + 1][mask ^ (1 << j)], f[i][j][mask] + 1);
}
int rem = min(mask, mask ^ (1 << j));
if(j > 0 && (a[i][j - 1] == '#') && (mask >> (j - 1) & 1 ^ 1)){
minimize(f[i][j + 1][rem], f[i][j][mask]);
} else{
minimize(f[i][j + 1][rem], f[i][j][mask] + 1);
}
} else{
if(mask >> j & 1){
minimize(f[i][j + 1][mask ^ (1 << j)], f[i][j][mask]);
} else{
minimize(f[i][j + 1][mask], f[i][j][mask]);
}
}
}
}
for(int mask = 0; mask < (1 << M); ++mask){
f[i + 1][0][mask] = f[i][M][mask];
}
}
int ans = inf;
for(int mask = 0; mask < (1 << M); ++mask){
ans = min(ans, f[N][0][mask]);
}
cout << ans << '\n';
return 0;
}
