// File U.cpp created on 09.08.2025 at 00:46:23
#include <bits/stdc++.h>
using i64 = long long;
#ifdef DEBUG
#include "/home/ahmetalp/Desktop/Workplace/debug.h"
#else
#define debug(...) void(23)
#endif
constexpr int max_N = int(4000) + 5;
int N, M;
char A[max_N][max_N];
bool vis[max_N][max_N];
constexpr int dx[] = {0, +1, 0, -1};
constexpr int dy[] = {+1, 0, -1, 0};
int main() {
std::ios::sync_with_stdio(false);
std::cin.tie(nullptr);
std::cin >> N >> M;
for (int i = 0; i < N; ++i) {
for (int j = 0; j < M; ++j) {
std::cin >> A[i][j];
}
}
std::queue<std::pair<int, int>> que;
vis[0][0] = true;
que.emplace(0, 0);
for (int t = 1; ; ++t) {
std::vector<std::pair<int, int>> vec;
while (!que.empty()) {
auto[x, y] = que.front();
que.pop();
for (int d = 0; d < 4; ++d) {
int nx = x + dx[d], ny = y + dy[d];
if (0 <= nx && nx < N && 0 <= ny && ny < M && vis[nx][ny] == false && A[nx][ny] != '.') {
if (A[nx][ny] == A[x][y]) {
vis[nx][ny] = true;
que.emplace(nx, ny);
} else {
vis[nx][ny] = true;
vec.emplace_back(nx, ny);
}
}
}
}
if (vec.empty()) {
std::cout << t << '\n';
break;
}
for (auto[x, y] : vec) {
que.emplace(x, y);
}
}
return 0;
}
