#include <cstdio>
#include <cstring>
#include <iostream>
#define MN 188

int n, m;
char c[MN][MN];
int f[MN][MN][MN][2], g[MN][MN][MN][2];
int sum[MN][MN];

int S(int a, int b, int c, int d) {return sum[c][d] - sum[a - 1][d] - sum[c][b - 1] + sum[a - 1][b - 1];}

int main()
{
	scanf("%d%d", &n, &m);
	for(int i = 1; i <= n; i++) scanf("%s", c[i] + 1);
	for(int i = 1; i <= n; i++)
		for(int j = 1; j <= m; j++)
			sum[i][j] = sum[i - 1][j] + sum[i][j - 1] - sum[i - 1][j - 1] + (c[i][j] == '#');
			
	for(int i = 1; i <= n; i++)
		for(int j = i; j <= n; j++)
		{
			int r = 0;
			for(int l = 1; l <= m; l++)
			{
				r = std::max(r, l - 1);
				while(r < m && (S(i, l, j, r + 1) == (j - i + 1) * (r + 1 - l + 1) || S(i, l, j, r + 1) == 0)) r++;
				f[i][j][l][0] = r;
			}
		}
			
	for(int i = 1; i <= m; i++)
		for(int j = i; j <= m; j++)
		{
			int r = 0;
			for(int l = 1; l <= n; l++)
			{
				r = std::max(r, l - 1);
				while(r < n && (S(l, i, r + 1, j) == (r + 1 - l + 1) * (j - i + 1) || S(l, i, r + 1, j) == 0)) r++;
				g[i][j][l][0] = r;
			}
		}
		
	if(f[1][n][1][0] == m) return 0 * puts("0");
	
	for(int o = 1; o <= 16; o++)
	{
		for(int i = 1; i <= n; i++)
			for(int j = i; j <= n; j++)
			{
				int r = 0;
				for(int l = 1; l <= m; l++)
				{
					r = std::max(r, f[i][j][std::min(m, f[i][j][l][(o & 1) ^ 1] + 1)][(o & 1) ^ 1]);
					while(r < m && g[l][r + 1][std::min(m, g[l][r + 1][i][(o & 1) ^ 1] + 1)][(o & 1) ^ 1] >= j) r++;
					f[i][j][l][o & 1] = r;
				}
			}
			
		for(int i = 1; i <= m; i++)
			for(int j = i; j <= m; j++)
			{
				int r = 0;
				for(int l = 1; l <= n; l++)
				{
					r = std::max(r, g[i][j][std::min(n, g[i][j][l][(o & 1) ^ 1] + 1)][(o & 1) ^ 1]);
					while(r < n && f[l][r + 1][std::min(n, f[l][r + 1][i][(o & 1) ^ 1] + 1)][(o & 1) ^ 1] >= j) r++;
					g[i][j][l][o & 1] = r;
				}
			}
		
		if(f[1][n][1][o & 1] == m) return 0 * printf("%d\n", o);
	}
}