#include <bits/stdc++.h>

using namespace std;
using ll = long long;
using VI = vector<int>;
using VL = vector<ll>;
using PII = std::pair<int, int>;
using PLL = std::pair<ll, ll>;

#define rep(i, n) for (int i = 0; i < (int)(n); i++)
#define repr(i, n) for (int i = (int)(n)-1; i >= 0; i--)
#define rep2(i, s, n) for (int i = (s); i < (int)(n); i++)
#define allpt(v) (v).begin(), (v).end()
#define allpt_c(v) (v).cbegin(), (v).cend()
#define allpt_r(v) (v).rbegin(), (v).rend()

const int mod = 1e9 + 7;
const string wsp = " ";
const string tb = "\t";
const string rt = "\n";

template <typename T>
void show1dvec(const vector<T> &v)
{
    if (v.size() == 0)
        return;
    int n = v.size() - 1;
    rep(i, n) cout << v[i] << wsp;
    cout << v[n] << rt;
    return;
}

template <typename T>
void show2dvec(const vector<vector<T>> &v)
{
    int n = v.size();
    rep(i, n) show1dvec(v[i]);
}

template <typename T, typename S>
void show1dpair(const vector<pair<T, S>> &v)
{
    int n = v.size();
    rep(i, n) cout << v[i].first << wsp << v[i].second << rt;
    return;
}

template <typename T, typename S>
void pairzip(const vector<pair<T, S>> &v, vector<T> &t, vector<T> &s)
{
    int n = v.size();
    rep(i, n)
    {
        t.push_back(v[i].first);
        s.push_back(v[i].second);
    }
    return;
}

template <typename T>
void maxvec(vector<T> &v)
{
    T s = v[0];
    int n = v.size();
    rep(i, n - 1)
    {
        if (s > v[i + 1])
        {
            v[i + 1] = s;
        }
        s = v[i + 1];
    }
}

template <typename T, typename S>
bool myfind(T t, S s)
{
    return find(t.cbegin(), t.cend(), s) != t.cend();
}

void dfs(int y, int x, int h, int w, int c, vector<string> &field, vector<VI> &flipfield, char &ch)
{
    if (0 <= y && y < h && 0 <= x && x < w && field[y][x] == ch && flipfield[y][x] == -1)
    {
        flipfield[y][x] = c;
        dfs(y + 1, x, h, w, c, field, flipfield, ch);
        dfs(y - 1, x, h, w, c, field, flipfield, ch);
        dfs(y, x + 1, h, w, c, field, flipfield, ch);
        dfs(y, x - 1, h, w, c, field, flipfield, ch);
    }
}


int main()
{

#ifdef DEBUG
    cout << "DEBUG MODE" << endl;
    ifstream in("input.txt"); //for debug
    cin.rdbuf(in.rdbuf());    //for debug
#endif

    int h, w, c = 0;
    cin >> h >> w;
    char ch;
    vector<string> field(h);
    vector<VI> flipfield(h, VI(w, 0));

    rep(i, h) cin >> field[i];

    rep(j, w - 1)
    {
        if (field[0][j] != field[0][j + 1])
            flipfield[0][j + 1] = flipfield[0][j] + 1;
        else
            flipfield[0][j + 1] = flipfield[0][j];
    }
    rep(i, h - 1)
    {
        if (field[i][0] != field[i + 1][0])
            flipfield[i + 1][0] = flipfield[i][0] + 1;
        else
            flipfield[i + 1][0] = flipfield[i][0];
    }
    rep(i, h - 1) rep(j, w - 1)
    {
        int a, b;
        a = flipfield[i + 1][j];
        b = flipfield[i][j + 1];
        if (field[i + 1][j] != field[i + 1][j + 1])
            a++;
        if (field[i][j + 1] != field[i + 1][j + 1])
            b++;
        flipfield[i + 1][j + 1] = min(a, b);
    }

    int ans = (flipfield[h - 1][w - 1] + 1) / 2;
    if (field[0][0] == '#' && field[h - 1][w - 1] == '#') ans++;
    cout << ans << rt;
    return 0;
}


Measured memory_bytes: 256