#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

#define rd_init() char*rp=({char*mmap();mmap(0l,1l<<25,1,2,0,0ll);})
#define rd() ({int _v=0,_c;while(_c=*rp++-48,_c>=0)_v=_v*10+_c;_v;})
#define wt(v) ({unsigned _z=v;do*--wp=_z%10+48;while(_z/=10);})
#define wt1(v) ({char wbuf[64],*wp=wbuf+sizeof wbuf;wt(v);write(1,wp,wbuf+sizeof wbuf-wp);})
#define rep(v,e) for(typeof(e) v=0;v<e;++v)
#define rrep(v,e) for(typeof(e) v=e;v--;)
#define chmin(v,a) (v=v<=a?v:a)

int a[2001][2001];
char c[2001][2001];
int d[2001];

int f1(char*rp,int h,int w){
	int n=0;
	rrep(y,h){
		rrep(x,w){
			if(*rp++=='#'){
				int b=a[y+0][x+1];
				chmin(b,a[y+1][x+0]);
				chmin(b,a[y+1][x+1]);
				a[y][x]=b+1;
				n+=1;
			}
		}
		++rp;
	}
	return n;
}

int f2(int h,int w,int n){
	int ok=1,ng=h+1;
	while(ok+1<ng){
		rep(y,2001){
			rep(x,2001){
				c[y][x]=0;
			}
		}
		rep(x,2001){
			d[x]=0;
		}
		int k=ok+ng>>1;
		int m=n;
		rep(y,h){
			int e=0;
			rep(x,w){
				if(a[y][x]>=k){
					c[y][x]+=1;
					c[y][x+k]-=1;
					c[y+k][x]-=1;
					c[y+k][x+k]+=1;
				}
				e+=d[x]+=c[y][x];
				m-=e>0;
			}
		}
		if(m){
			ng=k;
		}else{
			ok=k;
		}
	}
	return ok;
}

int main(){
	rd_init();
	int h=rd();
	int w=rd();
	int z=f2(h,w,f1(rp,h,w));
	wt1(z);
	_exit(0);
}