/*{{{*/
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<cmath>
#include<algorithm>
#include<string>
#include<iostream>
#include<sstream>
#include<set>
#include<map>
#include<queue>
#include<bitset>
#include<vector>
#include<limits.h>
#include<assert.h>
#define SZ(X) ((int)(X).size())
#define ALL(X) (X).begin(), (X).end()
#define REP(I, N) for (int I = 0; I < (N); ++I)
#define REPP(I, A, B) for (int I = (A); I < (B); ++I)
#define FOR(I, A, B) for (int I = (A); I <= (B); ++I)
#define FORS(I, S) for (int I = 0; S[I]; ++I)
#define RS(X) scanf("%s", (X))
#define SORT_UNIQUE(c) (sort(c.begin(),c.end()), c.resize(distance(c.begin(),unique(c.begin(),c.end()))))
#define GET_POS(c,x) (lower_bound(c.begin(),c.end(),x)-c.begin())
#define CASET int ___T; scanf("%d", &___T); for(int cs=1;cs<=___T;cs++)
#define MP make_pair
#define PB push_back
#define MS0(X) memset((X), 0, sizeof((X)))
#define MS1(X) memset((X), -1, sizeof((X)))
#define LEN(X) strlen(X)
#define F first
#define S second
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
typedef long double LD;
typedef pair<int,int> PII;
typedef vector<int> VI;
typedef vector<LL> VL;
typedef vector<PII> VPII;
typedef pair<LL,LL> PLL;
typedef vector<PLL> VPLL;
template<class T> void _R(T &x) { cin >> x; }
void _R(int &x) { scanf("%d", &x); }
void _R(LL &x) { scanf("%lld", &x); }
void _R(double &x) { scanf("%lf", &x); }
void _R(char &x) { scanf(" %c", &x); }
void _R(char *x) { scanf("%s", x); }
void R() {}
template<class T, class... U> void R(T &head, U &... tail) { _R(head); R(tail...); }
template<class T> void _W(const T &x) { cout << x; }
void _W(const int &x) { printf("%d", x); }
void _W(const LL &x) { printf("%lld", x); }
void _W(const double &x) { printf("%.16f", x); }
void _W(const char &x) { putchar(x); }
void _W(const char *x) { printf("%s", x); }
template<class T,class U> void _W(const pair<T,U> &x) {_W(x.F); putchar(' '); _W(x.S);}
template<class T> void _W(const vector<T> &x) { for (auto i = x.begin(); i != x.end(); _W(*i++)) if (i != x.cbegin()) putchar(' '); }
void W() {}
template<class T, class... U> void W(const T &head, const U &... tail) { _W(head); putchar(sizeof...(tail) ? ' ' : '\n'); W(tail...); }
#ifdef HOME
 #define DEBUG(...) {printf("# ");printf(__VA_ARGS__);puts("");}
#else
 #define DEBUG(...)
#endif
int MOD = 1e9+7;
void ADD(LL& x,LL v){x=(x+v)%MOD;if(x<0)x+=MOD;}
/*}}}*/
const int SIZE = 188;
int N,M;
char s[SIZE][SIZE];
int dp0[SIZE][SIZE];
int get(int x1,int y1,int x2,int y2){
    x1--;y1--;
    return dp0[x2][y2]+dp0[x1][y1]-dp0[x1][y2]-dp0[x2][y1];
}
bool same(int x1,int y1,int x2,int y2){
    int v=get(x1,y1,x2,y2);
    return !v||v==(x2-x1+1)*(y2-y1+1);
}
template <class T>
void maa(T& x,T y){
    if(x<y)x=y;
}
template <class T>
void mii(T& x,T y){
    if(x>y)x=y;
}
unsigned char dp[SIZE][SIZE][SIZE][20];
int main(){
    R(N,M);
    FOR(i,1,N)RS(s[i]+1);
    FOR(i,1,N)FOR(j,1,M){
        dp0[i][j]=dp0[i][j-1]+dp0[i-1][j]-dp0[i-1][j-1]+(s[i][j]=='#');
    }
    if(same(1,1,N,M)){
        W(0);
        return 0;
    }
    for(int i=M;i>0;i--){
        FOR(lo,1,N)FOR(hi,lo,N){
            dp[i][lo][hi][0]=i-1;
            if(same(lo,i,hi,i)){
                int ll=i,rr=M;
                while(ll<rr){
                    int mm=(ll+rr+1)/2;
                    if(same(lo,i,hi,mm))ll=mm;
                    else rr=mm-1;
                }
                dp[i][lo][hi][0]=ll;
            }
        }
    }
    FOR(lv,1,20){
        for(int i=M;i>0;i--){
            FOR(lo,1,N)FOR(hi,lo,N){
                int far=dp[i][lo][hi][lv-1];
                if(far<M)far=dp[far+1][lo][hi][lv-1];
                dp[i][lo][hi][lv]=far;
                if(lo!=hi){
                    /*
                    REPP(k,lo,hi){
                        maa(dp[i][lo][hi][lv],(unsigned char)min(dp[i][lo][k][lv-1],dp[i][k+1][hi][lv-1]));
                    }
                    continue;
                    */
                    int ll=lo,rr=hi-1;
                    while(ll<rr){
                        int mm=(ll+rr)/2;
                        int v1=dp[i][lo][mm][lv-1];
                        int v2=dp[i][mm+1][hi][lv-1];
                        if(v1>v2){
                            ll=mm+1;
                        }
                        else rr=mm;
                    }
                    maa(dp[i][lo][hi][lv],(unsigned char)(min(dp[i][lo][ll][lv-1],dp[i][ll+1][hi][lv-1])));
                    if(ll>lo){
                        ll--;
                        maa(dp[i][lo][hi][lv],(unsigned char)(min(dp[i][lo][ll][lv-1],dp[i][ll+1][hi][lv-1])));
                        ll++;
                    }
                    if(ll+1<hi){
                        ll++;
                        maa(dp[i][lo][hi][lv],(unsigned char)(min(dp[i][lo][ll][lv-1],dp[i][ll+1][hi][lv-1])));
                        ll--;
                    }
                }
            }
        }
        if(dp[1][1][N][lv]>=M){
            W(lv);
            return 0;
        }
    }
    return 0;
}


Measured memory_bytes: 128768