#define DEBUG 0

#include <bits/stdc++.h>
using namespace std;

#if DEBUG
// basic debugging macros
int __i__,__j__;
#define printLine(l) for(__i__=0;__i__<l;__i__++){cout<<"-";}cout<<endl
#define printLine2(l,c) for(__i__=0;__i__<l;__i__++){cout<<c;}cout<<endl
#define printVar(n) cout<<#n<<": "<<n<<endl
#define printArr(a,l) cout<<#a<<": ";for(__i__=0;__i__<l;__i__++){cout<<a[__i__]<<" ";}cout<<endl
#define print2dArr(a,r,c) cout<<#a<<":\n";for(__i__=0;__i__<r;__i__++){for(__j__=0;__j__<c;__j__++){cout<<a[__i__][__j__]<<" ";}cout<<endl;}
#define print2dArr2(a,r,c,l) cout<<#a<<":\n";for(__i__=0;__i__<r;__i__++){for(__j__=0;__j__<c;__j__++){cout<<setw(l)<<setfill(' ')<<a[__i__][__j__]<<" ";}cout<<endl;}

// advanced debugging class
// debug 1,2,'A',"test";
class _Debug {
    public:
        template<typename T>
        _Debug& operator,(T val) {
            cout << val << endl;
            return *this;
        }
};
#define debug _Debug(),
#else
#define printLine(l)
#define printLine2(l,c)
#define printVar(n)
#define printArr(a,l)
#define print2dArr(a,r,c)
#define print2dArr2(a,r,c,l)
#define debug
#endif

// define
#define MAX_VAL 999999999
#define MAX_VAL_2 999999999999999999LL
#define EPS 1e-6
#define mp make_pair
#define pb push_back

// typedef
typedef unsigned int UI;
typedef long long int LLI;
typedef unsigned long long int ULLI;
typedef unsigned short int US;
typedef pair<int,int> pii;
typedef pair<LLI,LLI> plli;
typedef vector<int> vi;
typedef vector<LLI> vlli;
typedef vector<pii> vpii;
typedef vector<plli> vplli;

// ---------- END OF TEMPLATE ----------

char grid[185][185];
int pre[186][186];
int temp[185],besth[185][185][185],bestv[185][185][185];
int query(int l,int r,int u,int d) {
    return pre[d+1][r+1]-pre[u][r+1]-pre[d+1][l]+pre[u][l];
}
int main() {
    int i,j;
    int H,W;
    scanf("%d %d",&H,&W);
    for (i = 0; i < H; i++) {
        for (j = 0; j < W; j++) {
            scanf(" %c",&grid[i][j]);
            pre[i+1][j+1] = pre[i+1][j]+pre[i][j+1]-pre[i][j]+(grid[i][j] == '#');
        }
    }

    int k;
    for (i = 0; i < H; i++) {
        for (j = i; j < H; j++) {
            for (k = 0; k < W; k++) {
                int l = 0,r = k+1;
                while (l < r) {
                    int m = (l+r) / 2;
                    int x = query(m,k,i,j);
                    if (((grid[j][k] == '#') && (x == (j-i+1)*(k-m+1))) || ((grid[j][k] == '.') && (x == 0))) r = m;
                    else l = m+1;
                }
                besth[i][j][k] = l;
            }
        }
    }
    for (i = 0; i < W; i++) {
        for (j = i; j < W; j++) {
            for (k = 0; k < H; k++) {
                int l = 0,r = k+1;
                while (l < r) {
                    int m = (l+r) / 2;
                    if (besth[m][k][j] <= i) r = m;
                    else l = m+1;
                }
                bestv[i][j][k] = l;
            }
        }
    }
    int ans = 0;
    while (besth[0][H-1][W-1] > 0) {
        ans++;
        for (i = 0; i < H; i++) {
            for (j = i; j < H; j++) {
                for (k = W-1; k >= 0; k--) {
                    if (besth[i][j][k] > 0) besth[i][j][k] = besth[i][j][besth[i][j][k]-1];
                }
            }
        }
        for (i = 0; i < W; i++) {
            for (j = i; j < W; j++) {
                for (k = H-1; k >= 0; k--) {
                    if (bestv[i][j][k] > 0) bestv[i][j][k] = bestv[i][j][bestv[i][j][k]-1];
                    int l = 0,r = bestv[i][j][k];
                    while (l < r) {
                        int m = (l+r) / 2;
                        if (besth[m][k][j] <= i) r = m;
                        else l = m+1;
                    }
                    bestv[i][j][k] = l;
                }
            }
        }
        for (i = 0; i < H; i++) {
            for (j = i; j < H; j++) {
                for (k = W-1; k >= 0; k--) {
                    int l = 0,r = besth[i][j][k];
                    while (l < r) {
                        int m = (l+r) / 2;
                        if (bestv[m][k][j] <= i) r = m;
                        else l = m+1;
                    }
                    besth[i][j][k] = l;
                }
            }
        }
    }
    printf("%d\n",ans);

    return 0;
}



Measured memory_bytes: 49536