S[3002][3002],H,W,x,y,z,d;
main(t){
	scanf("%d%d ",&H,&W);
	for(y=1;y<=H;++y)for(x=1;x<=W+1;++x){
		S[y][x]=getchar()=='#'?1e9:0;
	}
	for(y=1;y<=H;++y)for(x=1;x<=W;++x){
		t=S[y][x-1];
		for(d=-1;d<=1;++d){
			t=fmin(t,S[y-1][x+d]);
		}
		S[y][x]=fmin(S[y][x],t+1);
	}
	for(y=H;y>0;--y)for(x=W;x>0;--x){
		t=S[y][x+1];
		for(d=-1;d<=1;++d){
			t=fmin(t,S[y+1][x+d]);
		}
		z=fmax(z,S[y][x]=fmin(S[y][x],t+1));
	}
	printf("%d",z);
}