#include <bits/stdc++.h>
using namespace std;
#define int long long
#define inf 0x3F3F3F3F
int A[4] = {0, 0, -1, 1};
int B[4] = {-1, 1, 0, 0};
signed main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
int n, m;
cin >> n >> m;
char a[n][m];
int d[n][m];
for (int i = 0; i < n; i++) for (int j = 0; j < m; j++) cin >> a[i][j], d[i][j] = inf;
d[0][0] = 1;
assert(a[0][0] == a[n - 1][m - 1]);
deque<array<int, 2>> q;
q.push_back({0, 0});
int res = 0;
while (!q.empty())
{
array<int, 2> x = q.front();
q.pop_front();
for (int k = 0; k < 4; k++)
{
int X = x[0] + A[k], Y = x[1] + B[k];
if (min(X, Y) < 0 || X >= n || Y >= m || a[X][Y] == '.') continue;
if (a[X][Y] == a[x[0]][x[1]] && d[X][Y] > d[x[0]][x[1]])
{
d[X][Y] = d[x[0]][x[1]];
q.push_front({X, Y});
}
else if (a[X][Y] != a[x[0]][x[1]] && d[X][Y] > d[x[0]][x[1]] + 1)
{
d[X][Y] = d[x[0]][x[1]] + 1;
q.push_back({X, Y});
}
}
res = max(res, d[x[0]][x[1]]);
}
cout << res << '\n';
}
