#include<bits/stdc++.h>

#define maxn 2005

#define mod 1000000007

using namespace std;

typedef long long ll;

ll n,m;

char mp[maxn][maxn];

ll d[maxn][maxn];

ll q[maxn][maxn];

void solve(){

    ll mx=;

    for(int i=;i<=n;i++){

        stack<ll> e;

        e.push();

        for(int j=;j<=m+;j++){

            while(q[i][j]<q[i][e.top()]){

                ll dex=e.top();

                e.pop();

                ll r=j-e.top()-;

                mx=max(mx,min(r,q[i][dex]));

            }

            e.push(j);

        }

    }

    printf("%lld",mx);

}

int main(){

    scanf("%lld%lld",&n,&m);

    for(int i=;i<=n;i++){

        scanf("%s",mp[i]+);

    }

    for(int i=;i<=n;i++){

        for(int j=;j<=m;j++){

            if(mp[i][j]=='') d[i][j]=;

            else d[i][j]=;

        }

    }

    for(int i=;i<=n;i++){

        for(int j=;j<=m;j++){

            if((d[i][j]&&d[i-][j]==)||(d[i][j]==&&d[i-][j])) q[i][j]=q[i-][j]+;

            else q[i][j]=;

         }

    }

    solve();

    return ;

}