#include <bits/stdc++.h>
using namespace std;
#include <atcoder/all>
using namespace atcoder;
using ll = long long;
using vll = vector<ll>;
using vvll = vector<vll>;
using vvvll = vector<vvll>;
using vvvvll = vector<vvvll>;
using vvvvvll = vector<vvvvll>;
#define all(A) A.begin(),A.end()
#define rep(i, n) for (ll i = 0; i < (ll) (n); i++)
template<class T>
bool chmax(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p < q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}

template<class T>
bool chmin(T& p, T q, bool C = 1) {
    if (C == 0 && p == q) {
        return 1;
    }
    if (p > q) {
        p = q;
        return 1;
    }
    else {
        return 0;
    }
}

ll gcd(ll(a), ll(b)) {
    if (a == 0)return b;
    if (b == 0)return a;
    ll c = a;
    while (a % b != 0) {
        c = a % b;
        a = b;
        b = c;
    }
    return b;
}
ll sqrtz(ll N) {
    ll L = max(0ll,ll(sqrt(N)-20));
    ll R = sqrt(N) + 20;
    while (abs(R - L) > 1) {
        ll mid = (R + L) / 2;
        if (mid * mid <= N)L = mid;
        else R = mid;
    }
    return L;

}

ll inva(ll N, ll M) {
    ll mod = M;
    ll a = N, b = mod, c = 1, d = 0;
    while (b > 0) {
        ll t = a / b;
        a -= t * b;
        swap(a, b);
        c -= t * d;
        swap(c, d);
    }
    c %= mod;
    if (c < 0)c += mod;
    return c;

}

const ll mod=998244353;
ll modPow(long long a, long long n, long long p=mod) {
    if (n == 0) return 1; // 0乗にも対応する場合
    if (n == 1) return a % p;
    if (n % 2 == 1) return (a * modPow(a, n - 1, p)) % p;
    long long t = modPow(a, n / 2, p);
    return (t * t) % p;
}

using mint=modint998244353;
mint sum(mint a,mint d,ll n){
    if(n<0)return 0;
    return (a+(n-1)*d)/2*mint(n);
}

int main() {

    cin.tie(nullptr);
    ios::sync_with_stdio(false);
    
    ll H,W;
    cin>>H>>W;
    vector<string> SS(H);
    rep(h,H)cin>>SS[h];
    ll L=0,R=min(H,W)+1;
    while(R-L>1){
        ll M=(R+L)/2;
        //cout<<L<<" "<<R<<endl;
        vvll D(H+1,vll(W+1,0));
        rep(h,H){
            rep(w,W){
                if(SS[h][w]=='#')D[h+1][w+1]++;
            }
        }
        rep(h,H)rep(w,W+1)D[h+1][w]+=D[h][w];
        rep(h,H+1)rep(w,W)D[h][w+1]+=D[h][w];
        vvll S(H+1,vll(W+1,0));
        rep(h,H){
            rep(w,W){
                if(h+M>H||w+M>W)continue;
                if(D[h+M][w+M]-D[h+M][w]-D[h][w+M]+D[h][w]==M*M){
                    S[h][w]++;
                    S[h+M][w+M]++;
                    S[h+M][w]--;
                    S[h][w+M]--;
                }
            }
        }
        bool OK=1;
        rep(h,H)rep(w,W+1)S[h+1][w]+=S[h][w];
        rep(h,H+1)rep(w,W)S[h][w+1]+=S[h][w];
        rep(h,H)rep(w,W){
            if(SS[h][w]=='#'&&S[h][w]==0)OK=0;
        }
        //cout<<OK<<endl;
        (OK?L:R)=M;
    }
    cout<<L<<endl;
}