#include <iostream>
#define maxn 202
using namespace std;
string s[maxn];
int dp[2][maxn][maxn][maxn];
int main(){
	int n, m;
	cin >> n >> m;
	for (int i = 1; i <= n; ++ i)
		cin >> s[i];
	for (int i = n; i; -- i)
		for (int j = i; j <= n; ++ j)
			for (int x = m - 1; x >= 0; -- x){
				if (i == j){
					if (x == m - 1)
						dp[0][i][j][x] = m;
					else
						dp[0][i][j][x] = s[i][x] == s[i][x + 1] ? dp[0][i][j][x + 1] : x + 1;
				}
				else
					dp[0][i][j][x] = s[i][x] == s[i + 1][x] ? min(dp[0][i][i][x], dp[0][i + 1][j][x]) : x;
			}
	for (int ans = 0, t = 0; ; ++ ans, t ^= 1){
		if (dp[t][1][n][0] == m){
			cout << ans << endl;
			return 0;
		}
		for (int i = n; i; -- i)
			for (int j = i; j <= n; ++ j)
				for (int x = m - 1; x >= 0; -- x){
					dp[t ^ 1][i][j][x] = dp[t][i][j][x] == m ? m : dp[t][i][j][dp[t][i][j][x]];
					if (i < j){
						int l = i, r = j - 1;
						while (l <= r){
							int m = l + r >> 1;
							if (dp[t][i][m][x] <= dp[t][m + 1][j][x]){
								dp[t ^ 1][i][j][x] = max(dp[t ^ 1][i][j][x], dp[t][i][m][x]);
								r = m - 1;
							}
							else
								l = m + 1;
						}
						l = i, r = j - 1;
						while (l <= r){
							int m = l + r >> 1;
							if (dp[t][i][m][x] > dp[t][m + 1][j][x]){
								dp[t ^ 1][i][j][x] = max(dp[t ^ 1][i][j][x], dp[t][m + 1][j][x]);
								l = m + 1;
							}
							else
								r = m - 1;
						}
					}
				}
	}
}
