ll@h,@w,a[h+1][w+1]{},c[][]{},d[w],n=0;
rrep(y,h){
	rrep(x,w){
		char@s;
		if(s=='#'){
			a[y][x]=min(a[y][x+1],a[y+1][x],a[y+1][x+1])+1;
			n+=1;
		}
	}
}
ll ok=1,ng=h+1;
while(ok+1<ng){
	d[0..w-1]=0;
	ll k=ok+ng>>1;
	ll m=n;
	rep(y,h){
		ll 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];
			c[y][x]=0;
			m-=e>0;
		}
	}
	(m?ng:ok)=k;
}
wt(ok);